diff --git a/android/app/src/main/java/com/zulipmobile/notifications/FcmMessage.kt b/android/app/src/main/java/com/zulipmobile/notifications/FcmMessage.kt
index 760f51c9db1..d5fbc48a9c4 100644
--- a/android/app/src/main/java/com/zulipmobile/notifications/FcmMessage.kt
+++ b/android/app/src/main/java/com/zulipmobile/notifications/FcmMessage.kt
@@ -197,10 +197,12 @@ private fun extractIdentity(data: Map<String, String>): Identity =
         // use this as a substitute for `user_id` when that's missing...
         // but it'd be inherently buggy, and the bug it'd introduce seems
         // likely to affect more users than the bug it'd fix.  So just ignore.
+        // TODO(server-2.0): Delete this comment.
         // (data["user"] ignored)
 
         // `user_id` was added in server version 2.1.0 (released 2019-12-12;
         // commit 447a517e6, PR #12172.)
+        // TODO(server-2.1): Require this.
         userId = data["user_id"]?.parseInt("user_id")
     )
 
diff --git a/android/app/src/test/java/com/zulipmobile/notifications/FcmMessageTest.kt b/android/app/src/test/java/com/zulipmobile/notifications/FcmMessageTest.kt
index 91c5377353b..a875843ef84 100644
--- a/android/app/src/test/java/com/zulipmobile/notifications/FcmMessageTest.kt
+++ b/android/app/src/test/java/com/zulipmobile/notifications/FcmMessageTest.kt
@@ -198,6 +198,7 @@ class RemoveFcmMessageTest : FcmMessageTestBase() {
         ))
 
         /// The Zulip server before v2.0 sends this form (plus some irrelevant fields).
+        // TODO(server-2.0): Drop this, and the logic it tests.
         val singular = base.plus(sequenceOf(
             "zulip_message_id" to "123"
         ))
diff --git a/src/__tests__/lib/exampleData.js b/src/__tests__/lib/exampleData.js
index e4688e2b060..1bb39e865e6 100644
--- a/src/__tests__/lib/exampleData.js
+++ b/src/__tests__/lib/exampleData.js
@@ -328,7 +328,6 @@ const messagePropertiesFromSender = (user: User) => {
   const { user_id: sender_id, email: sender_email, full_name: sender_full_name } = user;
 
   return deepFreeze({
-    sender_domain: '',
     avatar_url: user.avatar_url,
     client: 'ExampleClient',
     gravatar_hash: 'd3adb33f',
@@ -714,8 +713,8 @@ export const action = Object.freeze({
     anchor: 0,
     numBefore: 50,
     numAfter: 50,
-    foundNewest: undefined,
-    foundOldest: undefined,
+    foundNewest: false,
+    foundOldest: false,
     ownUserId: selfUser.user_id,
   }): MessageFetchCompleteAction),
   // If a given action is only relevant to a single test file, no need to
diff --git a/src/account/accountsSelectors.js b/src/account/accountsSelectors.js
index ffa6f1295ec..6f817b8c7c8 100644
--- a/src/account/accountsSelectors.js
+++ b/src/account/accountsSelectors.js
@@ -89,8 +89,22 @@ export const getAccount = (state: PerAccountState): Account => {
 /** The realm URL for this account. */
 export const getRealmUrl = (state: PerAccountState): URL => getAccount(state).realm;
 
-/** The Zulip server version for this account. */
-export const getZulipVersion = (state: PerAccountState): ZulipVersion => {
+/**
+ * The Zulip server version for this account.
+ *
+ * Prefer `getZulipFeatureLevel`, which is finer-grained, for most uses.
+ * This function is useful for logging, for user-facing information, and for
+ * old releases (pre-3.0) where the feature level is always 0.
+ *
+ * This function assumes we have server data for this account, and if not it
+ * may throw.  If you want to call it from a context where we may not have
+ * server data, we can fix that; see implementation comment.
+ *
+ * If using this to condition behavior where we make a request to the
+ * server, note that there's fundamentally a race; for details, see
+ * `getZulipFeatureLevel`.
+ */
+export const getServerVersion = (state: PerAccountState): ZulipVersion => {
   const { zulipVersion } = getAccount(state);
   // This invariant will hold as long as we only call this function in a
   // context where we have server data.
@@ -107,13 +121,29 @@ export const getZulipVersion = (state: PerAccountState): ZulipVersion => {
   return zulipVersion;
 };
 
-/** The Zulip server feature level for this account. */
+/**
+ * The Zulip server feature level for this account.
+ *
+ * This function assumes we have server data for this account, and if not it
+ * may throw.  If you want to call it from a context where we may not have
+ * server data, we can fix that; see implementation comment.
+ *
+ * If using this to condition behavior where we make a request to the
+ * server, note that there's fundamentally a race: the server may have been
+ * upgraded since we last heard from it.  Generally we don't worry about
+ * this because (a) usually our behavior for old servers is fine for new
+ * servers (by design in the new server, because it's the same behavior old
+ * clients will have), and then this can only be a problem on downgrade,
+ * which is rare; (b) with the exception of initial fetch, almost all our
+ * requests come while we already have a live event queue which would tell
+ * us about an upgrade, so the race is quite narrow.
+ */
 export const getZulipFeatureLevel = (state: PerAccountState): number => {
   const { zulipFeatureLevel } = getAccount(state);
   // This invariant will hold as long as we only call this function in a
   // context where we have server data.
   //
-  // TODO(#5006): Much like getZulipVersion above.  This property is just a
+  // TODO(#5006): Much like getServerVersion above.  This property is just a
   //   bit newer: b058fa266, from 2020-09.
   invariant(zulipFeatureLevel !== null, 'zulipFeatureLevel must be non-null in PerAccountState');
   return zulipFeatureLevel;
@@ -178,12 +208,3 @@ export const getAuth = (state: PerAccountState): Auth => {
 export const getIdentity: Selector<Identity> = createSelector(getAuth, auth =>
   identityOfAuth(auth),
 );
-
-/**
- * The Zulip server version for this account, or null if unknown.
- *
- * See the `zulipVersion` property of `Account` for details on how this
- * information is kept up to date.
- */
-export const getServerVersion = (state: PerAccountState): ZulipVersion | null =>
-  getAccount(state).zulipVersion;
diff --git a/src/actionTypes.js b/src/actionTypes.js
index b9e9a934f62..50aafd18e5a 100644
--- a/src/actionTypes.js
+++ b/src/actionTypes.js
@@ -221,8 +221,8 @@ export type MessageFetchCompleteAction = {|
   anchor: number,
   numBefore: number,
   numAfter: number,
-  foundNewest: boolean | void,
-  foundOldest: boolean | void,
+  foundNewest: boolean,
+  foundOldest: boolean,
   ownUserId: UserId,
 |};
 
@@ -297,8 +297,8 @@ type EventSubscriptionUpdateAction = {|
   property: string,
   value: boolean | number | string,
 
+  // TODO(server-4.0): Delete these commented-out properties.
   // name: string, // exists pre-4.0, but expected to be removed soon
-
   // email: string, // gone in 4.0; was the user's own email, so never useful
 |};
 
diff --git a/src/api/eventTypes.js b/src/api/eventTypes.js
index 999a04400df..1a31d699cdd 100644
--- a/src/api/eventTypes.js
+++ b/src/api/eventTypes.js
@@ -145,6 +145,7 @@ export type UpdateMessageFlagsEvent = $ReadOnly<{|
 
   // Servers with feature level 32+ send `op`. Servers will eventually
   // stop sending `operation`; see #4238.
+  // TODO(server-4.0): Simplify to just `op`.
   operation?: 'add' | 'remove',
   op?: 'add' | 'remove',
 
@@ -168,6 +169,7 @@ export type RestartEvent = $ReadOnly<{|
   //
   // They have the same shape and meaning as the same-named fields in
   // the /server_settings and /register responses.
+  // TODO(server-4.0): Mark these as required.
   zulip_version?: string,
   zulip_feature_level?: number,
 |}>;
diff --git a/src/api/initialDataTypes.js b/src/api/initialDataTypes.js
index d6fff130c5d..709edc62184 100644
--- a/src/api/initialDataTypes.js
+++ b/src/api/initialDataTypes.js
@@ -30,10 +30,11 @@ export type InitialDataBase = $ReadOnly<{|
   // `fetch_event_types` and remove this comment.
 
   /**
-   * Added in server version 2.2, feature level 1.
+   * Added in server version 3.0, feature level 1.
    * Same meaning as in the server_settings response:
    * https://zulip.com/api/get-server-settings. See also the comment above.
    */
+  // TODO(server-3.0): Mark as required.
   zulip_feature_level?: number,
 
   /**
@@ -59,8 +60,8 @@ export type InitialDataMutedTopics = $ReadOnly<{|
   muted_topics: $ReadOnlyArray<MuteTuple>,
 |}>;
 
-/** Added in server version 4.0, feature level 48 */
 export type InitialDataMutedUsers = $ReadOnly<{|
+  /** (When absent, treat as empty.  Added in server version 4.0, feature level 48.) */
   muted_users?: $ReadOnlyArray<MutedUser>,
 |}>;
 
@@ -124,6 +125,7 @@ export type InitialDataRealmEmoji = $ReadOnly<{|
 export type RawInitialDataRealmFilters = $ReadOnly<{|
   // We still request this, since not all servers can provide the
   // newer `realm_linkifiers` format.
+  // TODO(server-4.0): Drop this.
   realm_filters?: $ReadOnlyArray<RealmFilter>,
 |}>;
 
@@ -173,14 +175,17 @@ export type InitialDataRealmUserGroups = $ReadOnly<{|
   /**
    * Absent in servers prior to v1.8.0-rc1~2711 (or thereabouts).
    */
+  // TODO(server-1.8): Mark as required.
   realm_user_groups?: $ReadOnlyArray<UserGroup>,
 |}>;
 
 export type InitialDataRecentPmConversations = $ReadOnly<{|
   // * Added in server commit 2.1-dev-384-g4c3c669b41.
+  //   TODO(server-2.1): Mark this required.  See MIN_RECENTPMS_SERVER_VERSION.
   // * `user_id` fields are sorted as of commit 2.2-dev-53-g405a529340, which
   //    was backported to 2.1.1-50-gd452ad31e0 -- meaning that they are _not_
   //    sorted in either v2.1.0 or v2.1.1.
+  // TODO(server-3.0): Simply say these are sorted.  ("2.2" became 3.0.)
   recent_private_conversations?: $ReadOnlyArray<RecentPrivateConversation>,
 |}>;
 
diff --git a/src/api/messages/getMessages.js b/src/api/messages/getMessages.js
index 712d201ec02..16631a9bb37 100644
--- a/src/api/messages/getMessages.js
+++ b/src/api/messages/getMessages.js
@@ -10,9 +10,9 @@ import { AvatarURL } from '../../utils/avatar';
 type ApiResponseMessages = {|
   ...$Exact<ApiResponseSuccess>,
   anchor: number,
-  found_anchor?: boolean,
-  found_newest?: boolean,
-  found_oldest?: boolean,
+  found_anchor: boolean,
+  found_newest: boolean,
+  found_oldest: boolean,
   messages: Message[],
 |};
 
@@ -25,6 +25,7 @@ type ApiResponseMessages = {|
 // We shouldn't have to rely on this format on servers at feature
 // level 2+; those newer servers include a top-level `user_id` field
 // in addition to the `user` object. See #4072.
+// TODO(server-3.0): Simplify this away.
 export type ServerReaction = $ReadOnly<{|
   ...$Diff<Reaction, {| user_id: mixed |}>,
   user: $ReadOnly<{|
@@ -83,11 +84,6 @@ const migrateResponse = (response, identity: Identity) => {
 
 /**
  * See https://zulip.com/api/get-messages
- *
- * These values exist only in Zulip 1.8 or newer:
- *   * found_anchor
- *   * found_newest
- *   * found_oldest
  */
 export default async (
   auth: Auth,
diff --git a/src/api/modelTypes.js b/src/api/modelTypes.js
index 6e0c3d13536..836666886da 100644
--- a/src/api/modelTypes.js
+++ b/src/api/modelTypes.js
@@ -37,6 +37,7 @@ export type RealmEmojiById = $ReadOnly<{|
 /**
  * The only way servers before feature level 54 represent linkifiers.
  */
+// TODO(server-4.0): Delete this.
 export type RealmFilter = [string, string, number];
 
 /**
@@ -52,8 +53,8 @@ export type RealmFilter = [string, string, number];
 //   `realm_linkifiers` format. (When doing so, also don't forget to
 //   change various variable and type definition names to be like
 //   `realm_linkifiers`.)
-// - When we drop support for servers older than 54, we can remove all
-//   our code that knows about the `realm_filters` format.
+// - TODO(server-4.0): When we drop support for servers older than 54, we
+//   can remove all our code that knows about the `realm_filters` format.
 export type RealmLinkifier = $ReadOnly<{|
   id: number,
   pattern: string,
@@ -108,6 +109,7 @@ export type User = $ReadOnly<{|
   full_name: string,
 
   // date_joined included since commit 372e9740a (in 1.9.0)
+  // TODO(server-1.9): mark this as required
   date_joined?: string,
 
   // is_active doesn't appear in `/register` responses -- instead,
@@ -155,7 +157,7 @@ export type User = $ReadOnly<{|
   // profile_data added in commit 02b845336 (in 1.8.0);
   // see also e3aed0f7b (in 2.0.0)
   // (This one doesn't appear in `/users` responses.)
-  profile_data?: empty, // TODO describe actual type
+  profile_data?: empty, // When we need this, describe its actual type.
 |}>;
 
 /**
@@ -177,6 +179,7 @@ export type CrossRealmBot = $ReadOnly<{|
   avatar_url: AvatarURL,
 
   // date_joined included since commit 58ee3fa8c (in 1.9.0)
+  // TODO(server-1.9): mark this as required
   date_joined?: string,
 
   email: string,
@@ -351,6 +354,7 @@ export type NarrowElement =
  //  * `group-pm-with` since 2.1-dev-1813-gb338fd130
  //  * `sender` since 2.1-dev-1812-gc067c155a
  //  * `pm-with` since 2.1-dev-1350-gd7b4de234
+ // TODO(server-2.1): Stop sending stream names or user emails here.
  | {| +operator: 'stream', +operand: string | number |} // stream ID
  | {| +operator: 'pm-with', +operand: string | $ReadOnlyArray<UserId> |}
  | {| +operator: 'sender', +operand: string | UserId |}
@@ -539,9 +543,6 @@ type MessageBase = $ReadOnly<{|
   match_content?: string,
   match_subject?: string,
 
-  /** Obsolete? Gone in server commit 1.6.0~1758 . */
-  sender_domain: string,
-
   /**
    * The `flags` story is a bit complicated:
    *  * Absent in `event.message` for a `message` event... but instead
@@ -598,7 +599,7 @@ export type PmMessage = $ReadOnly<{|
   // Notes from studying the server code:
   //  * Notes are primarily from the server as of 2020-04 at cb85763c7, but
   //    this logic is very stable; confirmed all points about behavior as of
-  //    1.8.0, too.
+  //    1.8.0 (from 2018-04), too.
   //
   //  * This field is ultimately computed (for both events and /messages
   //    results) in MessageDict.hydrate_recipient_info, with most of the
@@ -725,5 +726,6 @@ export type RecentPrivateConversation = $ReadOnly<{|
   max_message_id: number,
   // When received from the server, these are guaranteed to be sorted only after
   // 2.2-dev-53-g405a529340. To be safe, we always sort them on receipt.
+  // TODO(server-3.0): Stop sorting these ourselves.  ("2.2" became 3.0.)
   user_ids: $ReadOnlyArray<UserId>,
 |}>;
diff --git a/src/api/registerForEvents.js b/src/api/registerForEvents.js
index 8fc437dcb29..a158ec5e2d5 100644
--- a/src/api/registerForEvents.js
+++ b/src/api/registerForEvents.js
@@ -54,6 +54,8 @@ const transform = (rawInitialData: RawInitialData, auth: Auth): InitialData => (
   // Transform the newer `realm_linkifiers` format, if present, to the
   // older `realm_filters` format. We do the same transformation on
   // 'realm_linkifiers' events.
+  // TODO(server-4.0): Switch to new format, if we haven't already;
+  //   and drop conversion.
   realm_filters: rawInitialData.realm_linkifiers
     ? rawInitialData.realm_linkifiers.map(({ pattern, url_format, id }) => [
         pattern,
diff --git a/src/api/settings/getServerSettings.js b/src/api/settings/getServerSettings.js
index f0b34c867f2..2c8bbb515ff 100644
--- a/src/api/settings/getServerSettings.js
+++ b/src/api/settings/getServerSettings.js
@@ -26,6 +26,7 @@ export type ApiResponseServerSettings = {|
   ...$Exact<ApiResponseSuccess>,
   authentication_methods: AuthenticationMethods,
   // external_authentication_methods added for server v2.1
+  // TODO(server-2.1): Mark this as required; simplify downstream.
   external_authentication_methods?: ExternalAuthenticationMethod[],
   email_auth_enabled: boolean,
   push_notifications_enabled: boolean,
@@ -36,8 +37,9 @@ export type ApiResponseServerSettings = {|
   require_email_format_usernames: boolean,
   zulip_version: string,
 
-  // zulip_feature_level added for server v2.2, feature level 1
+  // zulip_feature_level added for server v3.0, feature level 1
   // See https://zulip.com/api/get-server-settings
+  // When absent, equivalent to 0.
   zulip_feature_level?: number,
 |};
 
diff --git a/src/api/streams/updateStream.js b/src/api/streams/updateStream.js
index ae9af9f7901..47d2a6bb66e 100644
--- a/src/api/streams/updateStream.js
+++ b/src/api/streams/updateStream.js
@@ -9,6 +9,7 @@ import { apiPatch } from '../apiFetch';
  */
 // TODO(#4659): Once we pass the feature level to API methods, this one
 //   should encapsulate a switch at feature level 64.  See its call sites.
+// TODO(server-4.0): Simplify that away.
 export default (
   auth: Auth,
   id: number,
diff --git a/src/api/subscriptions/setSubscriptionProperty.js b/src/api/subscriptions/setSubscriptionProperty.js
index b9e3639fdad..3539fced302 100644
--- a/src/api/subscriptions/setSubscriptionProperty.js
+++ b/src/api/subscriptions/setSubscriptionProperty.js
@@ -20,8 +20,8 @@ export default async (
         // the server the older, confusingly named property
         // 'in_home_view' with the opposite value.
         //
-        // TODO: 'is_muted' is said to be new in Zulip 2.1, released
-        // 2019-12-12. Switch to sending that, once we can.
+        // TODO(server-2.1): 'is_muted' is said to be new in Zulip 2.1,
+        //   released 2019-12-12.  Switch to sending that, once we can.
         ...(property === 'is_muted'
           ? { property: 'in_home_view', value: !value }
           : { property, value }),
diff --git a/src/api/transportTypes.js b/src/api/transportTypes.js
index 72072f0b4b9..ccd7fee64d9 100644
--- a/src/api/transportTypes.js
+++ b/src/api/transportTypes.js
@@ -61,6 +61,8 @@ export type ApiResponseSuccess = $ReadOnly<{
  * that value when connecting to old servers that don't provide an error
  * code.
  *
+ * TODO(server-1.7): Simplify this.
+ *
  * [1] Specifically at 1.7.0~2354 and ancestors, aka 9faa44af6^..709c3b50fc .
  *     See: https://github.com/zulip/zulip/commit/709c3b50fc
  *
diff --git a/src/caughtup/__tests__/caughtUpReducer-test.js b/src/caughtup/__tests__/caughtUpReducer-test.js
index dd07f4ba47b..193a34bf952 100644
--- a/src/caughtup/__tests__/caughtUpReducer-test.js
+++ b/src/caughtup/__tests__/caughtUpReducer-test.js
@@ -4,14 +4,7 @@ import deepFreeze from 'deep-freeze';
 import * as eg from '../../__tests__/lib/exampleData';
 import caughtUpReducer from '../caughtUpReducer';
 import { MESSAGE_FETCH_ERROR } from '../../actionConstants';
-import { LAST_MESSAGE_ANCHOR, FIRST_UNREAD_ANCHOR } from '../../anchor';
-import {
-  HOME_NARROW,
-  HOME_NARROW_STR,
-  ALL_PRIVATE_NARROW,
-  ALL_PRIVATE_NARROW_STR,
-  SEARCH_NARROW,
-} from '../../utils/narrow';
+import { HOME_NARROW, HOME_NARROW_STR, SEARCH_NARROW } from '../../utils/narrow';
 
 describe('caughtUpReducer', () => {
   describe('MESSAGE_FETCH_START', () => {
@@ -85,24 +78,13 @@ describe('caughtUpReducer', () => {
   });
 
   describe('MESSAGE_FETCH_COMPLETE', () => {
-    test('if messages received are less than requested then we are caught up', () => {
-      const initialState = deepFreeze({
-        [HOME_NARROW_STR]: {
-          older: false,
-          newer: false,
-        },
-      });
+    test('apply `foundNewest` and `foundOldest` when true', () => {
+      const initialState = deepFreeze({});
 
       const action = deepFreeze({
         ...eg.action.message_fetch_complete,
-        anchor: 1,
-        messages: [
-          eg.streamMessage({ id: 1 }),
-          eg.streamMessage({ id: 2 }),
-          eg.streamMessage({ id: 3 }),
-        ],
-        numBefore: 5,
-        numAfter: 5,
+        foundNewest: true,
+        foundOldest: true,
       });
 
       const expectedState = {
@@ -118,16 +100,13 @@ describe('caughtUpReducer', () => {
     });
 
     test('if fetched messages are from a search narrow, ignore them', () => {
-      const initialState = deepFreeze({
-        [HOME_NARROW_STR]: {
-          older: false,
-          newer: false,
-        },
-      });
+      const initialState = deepFreeze({});
 
       const action = deepFreeze({
         ...eg.action.message_fetch_complete,
         narrow: SEARCH_NARROW('some query'),
+        foundOldest: true,
+        foundNewest: true,
       });
 
       const newState = caughtUpReducer(initialState, action);
@@ -136,43 +115,7 @@ describe('caughtUpReducer', () => {
     });
   });
 
-  test('if messages received are requested amount we consider it not yet caught up', () => {
-    const initialState = deepFreeze({
-      [HOME_NARROW_STR]: {
-        older: false,
-        newer: false,
-      },
-    });
-
-    const action = deepFreeze({
-      ...eg.action.message_fetch_complete,
-      anchor: 3,
-      messages: [
-        eg.streamMessage({ id: 1 }),
-        eg.streamMessage({ id: 2 }),
-        eg.streamMessage({ id: 3 }),
-        eg.streamMessage({ id: 4 }),
-        eg.streamMessage({ id: 5 }),
-      ],
-      numBefore: 2,
-      numAfter: 2,
-      foundNewest: undefined,
-      foundOldest: undefined,
-    });
-
-    const expectedState = {
-      [HOME_NARROW_STR]: {
-        older: false,
-        newer: false,
-      },
-    };
-
-    const newState = caughtUpReducer(initialState, action);
-
-    expect(newState).toEqual(expectedState);
-  });
-
-  test('new results do not reset previous state', () => {
+  test('new false results do not reset previous true state', () => {
     const initialState = deepFreeze({
       [HOME_NARROW_STR]: {
         older: true,
@@ -182,266 +125,8 @@ describe('caughtUpReducer', () => {
 
     const action = deepFreeze({
       ...eg.action.message_fetch_complete,
-      anchor: 3,
-      messages: [
-        eg.streamMessage({ id: 1 }),
-        eg.streamMessage({ id: 2 }),
-        eg.streamMessage({ id: 3 }),
-        eg.streamMessage({ id: 4 }),
-        eg.streamMessage({ id: 5 }),
-      ],
-      numBefore: 2,
-      numAfter: 2,
-    });
-
-    const expectedState = {
-      [HOME_NARROW_STR]: {
-        older: true,
-        newer: true,
-      },
-    };
-
-    const newState = caughtUpReducer(initialState, action);
-
-    expect(newState).toEqual(expectedState);
-  });
-
-  test('when at first unread and before and after messages are as many as requested not yet caught up', () => {
-    const initialState = deepFreeze({});
-
-    const action = deepFreeze({
-      ...eg.action.message_fetch_complete,
-      anchor: FIRST_UNREAD_ANCHOR,
-      messages: [
-        eg.streamMessage({ id: 1, flags: ['read'] }),
-        eg.streamMessage({ id: 2, flags: ['read'] }),
-        eg.streamMessage({ id: 3, flags: ['read'] }),
-        eg.streamMessage({ id: 4, flags: [] }),
-        eg.streamMessage({ id: 5, flags: [] }),
-        eg.streamMessage({ id: 6, flags: [] }),
-        eg.streamMessage({ id: 7, flags: [] }),
-      ],
-      numBefore: 3,
-      numAfter: 3,
-    });
-
-    const expectedState = {
-      [HOME_NARROW_STR]: {
-        older: false,
-        newer: false,
-      },
-    };
-
-    const newState = caughtUpReducer(initialState, action);
-
-    expect(newState).toEqual(expectedState);
-  });
-
-  test('when at first unread and before messages are less than requested older is caught up', () => {
-    const initialState = deepFreeze({});
-
-    const action = deepFreeze({
-      ...eg.action.message_fetch_complete,
-      anchor: FIRST_UNREAD_ANCHOR,
-      messages: [
-        eg.streamMessage({ id: 1, flags: ['read'] }),
-        eg.streamMessage({ id: 2, flags: ['read'] }),
-        eg.streamMessage({ id: 3, flags: [] }),
-        eg.streamMessage({ id: 4, flags: [] }),
-        eg.streamMessage({ id: 5, flags: [] }),
-        eg.streamMessage({ id: 6, flags: [] }),
-      ],
-      numBefore: 3,
-      numAfter: 4,
-    });
-
-    const expectedState = {
-      [HOME_NARROW_STR]: {
-        older: true,
-        newer: false,
-      },
-    };
-
-    const newState = caughtUpReducer(initialState, action);
-
-    expect(newState).toEqual(expectedState);
-  });
-
-  test('when at first unread and after messages are less than requested newer is caught up', () => {
-    const initialState = deepFreeze({});
-
-    const action = deepFreeze({
-      ...eg.action.message_fetch_complete,
-      anchor: FIRST_UNREAD_ANCHOR,
-      messages: [
-        eg.streamMessage({ id: 1, flags: ['read'] }),
-        eg.streamMessage({ id: 2, flags: ['read'] }),
-        eg.streamMessage({ id: 3, flags: ['read'] }),
-        eg.streamMessage({ id: 4, flags: [] }),
-        eg.streamMessage({ id: 5, flags: [] }),
-        eg.streamMessage({ id: 6, flags: [] }),
-      ],
-      numBefore: 3,
-      numAfter: 4,
-    });
-
-    const expectedState = {
-      [HOME_NARROW_STR]: {
-        older: false,
-        newer: true,
-      },
-    };
-
-    const newState = caughtUpReducer(initialState, action);
-
-    expect(newState).toEqual(expectedState);
-  });
-
-  test('when at first unread and both before and after messages are less than requested older and newer are caught up', () => {
-    const initialState = deepFreeze({});
-
-    const action = deepFreeze({
-      ...eg.action.message_fetch_complete,
-      anchor: FIRST_UNREAD_ANCHOR,
-      messages: [
-        eg.streamMessage({ id: 1, flags: ['read'] }),
-        eg.streamMessage({ id: 2, flags: ['read'] }),
-        eg.streamMessage({ id: 3, flags: [] }),
-        eg.streamMessage({ id: 4, flags: [] }),
-        eg.streamMessage({ id: 5, flags: [] }),
-      ],
-      numBefore: 3,
-      numAfter: 4,
-    });
-
-    const expectedState = {
-      [HOME_NARROW_STR]: {
-        older: true,
-        newer: true,
-      },
-    };
-
-    const newState = caughtUpReducer(initialState, action);
-
-    expect(newState).toEqual(expectedState);
-  });
-
-  test('if requesting latest messages always newer is caught up', () => {
-    const initialState = deepFreeze({});
-
-    const action = deepFreeze({
-      ...eg.action.message_fetch_complete,
-      narrow: ALL_PRIVATE_NARROW,
-      anchor: LAST_MESSAGE_ANCHOR,
-      messages: [
-        eg.streamMessage({ id: 1 }),
-        eg.streamMessage({ id: 2 }),
-        eg.streamMessage({ id: 3 }),
-        eg.streamMessage({ id: 4 }),
-        eg.streamMessage({ id: 5 }),
-      ],
-      numBefore: 10,
-      numAfter: 0,
-    });
-
-    const expectedState = {
-      [ALL_PRIVATE_NARROW_STR]: {
-        older: true,
-        newer: true,
-      },
-    };
-
-    const newState = caughtUpReducer(initialState, action);
-
-    expect(newState).toEqual(expectedState);
-  });
-
-  describe('verify that server has send extra message before calculating adjustment', () => {
-    test('no adjustment is required if messages are less than or equal to requested', () => {
-      const initialState = deepFreeze({
-        [HOME_NARROW_STR]: {},
-      });
-
-      const action = deepFreeze({
-        ...eg.action.message_fetch_complete,
-        anchor: 6,
-        messages: [
-          eg.streamMessage({ id: 1 }),
-          eg.streamMessage({ id: 2 }),
-          eg.streamMessage({ id: 3 }),
-          eg.streamMessage({ id: 4 }),
-          eg.streamMessage({ id: 5 }),
-          eg.streamMessage({ id: 6 }),
-          eg.streamMessage({ id: 7 }),
-          eg.streamMessage({ id: 8 }),
-          eg.streamMessage({ id: 9 }),
-          eg.streamMessage({ id: 10 }),
-        ],
-        numBefore: 5,
-        numAfter: 5,
-      });
-
-      const expectedState = {
-        [HOME_NARROW_STR]: {
-          older: false,
-          newer: false,
-        },
-      };
-
-      const newState = caughtUpReducer(initialState, action);
-
-      expect(newState).toEqual(expectedState);
-    });
-
-    test('dynamically determine adjustment whenever required', () => {
-      const initialState = deepFreeze({
-        [HOME_NARROW_STR]: {},
-      });
-
-      const action = deepFreeze({
-        ...eg.action.message_fetch_complete,
-        anchor: 5,
-        messages: [
-          eg.streamMessage({ id: 0 }),
-          eg.streamMessage({ id: 1 }),
-          eg.streamMessage({ id: 2 }),
-          eg.streamMessage({ id: 3 }),
-          eg.streamMessage({ id: 4 }),
-          eg.streamMessage({ id: 5 }),
-          eg.streamMessage({ id: 6 }),
-          eg.streamMessage({ id: 7 }),
-          eg.streamMessage({ id: 8 }),
-          eg.streamMessage({ id: 9 }),
-          eg.streamMessage({ id: 10 }),
-        ],
-        numBefore: 5,
-        numAfter: 5,
-      });
-
-      const expectedState = {
-        [HOME_NARROW_STR]: {
-          older: false,
-          newer: false,
-        },
-      };
-
-      const newState = caughtUpReducer(initialState, action);
-
-      expect(newState).toEqual(expectedState);
-    });
-  });
-
-  test('if `foundNewest` and `foundOldest` are provided use them', () => {
-    const initialState = deepFreeze({});
-
-    const action = deepFreeze({
-      ...eg.action.message_fetch_complete,
-      anchor: 3,
-      messages: [],
-      numBefore: 2,
-      numAfter: 2,
-      foundNewest: true,
-      foundOldest: true,
+      foundOldest: false,
+      foundNewest: false,
     });
 
     const expectedState = {
diff --git a/src/caughtup/caughtUpReducer.js b/src/caughtup/caughtUpReducer.js
index 833e11a268c..859adf210f9 100644
--- a/src/caughtup/caughtUpReducer.js
+++ b/src/caughtup/caughtUpReducer.js
@@ -1,5 +1,5 @@
 /* @flow strict-local */
-import type { CaughtUp, CaughtUpState, Action } from '../types';
+import type { CaughtUpState, Action } from '../types';
 import {
   REALM_INIT,
   LOGOUT,
@@ -9,54 +9,12 @@ import {
   MESSAGE_FETCH_ERROR,
   MESSAGE_FETCH_COMPLETE,
 } from '../actionConstants';
-import { LAST_MESSAGE_ANCHOR, FIRST_UNREAD_ANCHOR } from '../anchor';
 import { NULL_OBJECT } from '../nullObjects';
 import { DEFAULT_CAUGHTUP } from './caughtUpSelectors';
 import { isSearchNarrow, keyFromNarrow } from '../utils/narrow';
 
 const initialState: CaughtUpState = NULL_OBJECT;
 
-/** Try to infer the caught-up state, when the server didn't tell us. */
-const legacyInferCaughtUp = (prevCaughtUp: CaughtUp | void, action) => {
-  if (action.anchor === LAST_MESSAGE_ANCHOR) {
-    return {
-      older: action.numBefore > action.messages.length,
-      newer: true,
-    };
-  }
-
-  let anchorIdx = -1;
-
-  if (action.anchor === FIRST_UNREAD_ANCHOR) {
-    anchorIdx = action.messages.findIndex(msg => !msg.flags || msg.flags.indexOf('read') === -1);
-  } else {
-    anchorIdx = action.messages.findIndex(msg => msg.id === action.anchor);
-  }
-
-  if (anchorIdx === -1) {
-    anchorIdx = action.messages.length;
-  }
-
-  const totalMessagesRequested = action.numBefore + action.numAfter;
-  // If we're requesting messages before the anchor, the server
-  // returns one less than we expect (to avoid duplicating the anchor)
-  // only do adjustment if messages are more than expected
-  const adjustment =
-    action.messages.length > totalMessagesRequested && action.numBefore > 0
-      ? -(action.messages.length - totalMessagesRequested)
-      : 0;
-
-  const caughtUpOlder = anchorIdx < action.numBefore;
-  const caughtUpNewer = action.messages.length - anchorIdx + adjustment < action.numAfter;
-
-  const { older: prevOlder, newer: prevNewer } = prevCaughtUp || DEFAULT_CAUGHTUP;
-
-  return {
-    older: prevOlder || caughtUpOlder,
-    newer: prevNewer || caughtUpNewer,
-  };
-};
-
 export default (state: CaughtUpState = initialState, action: Action): CaughtUpState => {
   switch (action.type) {
     case REALM_INIT:
@@ -90,20 +48,13 @@ export default (state: CaughtUpState = initialState, action: Action): CaughtUpSt
         return state;
       }
       const key = keyFromNarrow(action.narrow);
-      let caughtUp = undefined;
-      if (action.foundNewest !== undefined && action.foundOldest !== undefined) {
-        /* This should always be the case for Zulip Server v1.8 or newer. */
-        const { older: prevOlder, newer: prevNewer } = state[key] || DEFAULT_CAUGHTUP;
-        caughtUp = {
-          older: prevOlder || action.foundOldest,
-          newer: prevNewer || action.foundNewest,
-        };
-      } else {
-        caughtUp = legacyInferCaughtUp(state[key], action);
-      }
+      const { older: prevOlder, newer: prevNewer } = state[key] || DEFAULT_CAUGHTUP;
       return {
         ...state,
-        [key]: caughtUp,
+        [key]: {
+          older: prevOlder || action.foundOldest,
+          newer: prevNewer || action.foundNewest,
+        },
       };
     }
 
diff --git a/src/common/ServerCompatBanner.js b/src/common/ServerCompatBanner.js
index 136a9726199..e520ce5a49f 100644
--- a/src/common/ServerCompatBanner.js
+++ b/src/common/ServerCompatBanner.js
@@ -13,6 +13,11 @@ import { openLinkWithUserPreference } from '../utils/openLink';
 // The oldest version we currently support. Should match what we say at
 //   https://zulip.readthedocs.io/en/stable/overview/release-lifecycle.html#compatibility-and-upgrading.
 const minSupportedVersion = '2.1.0';
+// Notes on known breakage at older versions:
+//  * Before 1.8, the server doesn't send found_newest / found_oldest on
+//    fetching messages, and so `state.caughtUp` will never have truthy
+//    values.  This probably means annoying behavior in a message list,
+//    as we keep trying to fetch newer messages.
 
 type Props = $ReadOnly<{||}>;
 
@@ -31,7 +36,7 @@ export default function ServerCompatBanner(props: Props): Node {
 
   let visible = false;
   let text = '';
-  if (!zulipVersion || zulipVersion.isAtLeast(minSupportedVersion)) {
+  if (zulipVersion.isAtLeast(minSupportedVersion)) {
     // don't show
   } else if (hasDismissedServerCompatNotice) {
     // don't show
diff --git a/src/events/eventToAction.js b/src/events/eventToAction.js
index c7b09773559..fb38cbda619 100644
--- a/src/events/eventToAction.js
+++ b/src/events/eventToAction.js
@@ -128,6 +128,7 @@ export default (state: PerAccountState, event: $FlowFixMe): EventAction | null =
         // Before server feature level 13 (or if we didn't specify the
         // `bulk_message_deletion` client capability, which we do), this
         // event has `message_id` instead of `message_ids`.
+        // TODO(server-3.0): Simplify this.
         messageIds: event.message_ids ?? [event.message_id],
       };
 
@@ -286,6 +287,7 @@ export default (state: PerAccountState, event: $FlowFixMe): EventAction | null =
 
         // Servers with feature level 32+ send `op`. Servers will eventually
         // stop sending `operation`; see #4238.
+        // TODO(server-4.0): Simplify to just use `op`.
         op: event.op ?? event.operation,
 
         allMessages: state.messages,
diff --git a/src/message/fetchActions.js b/src/message/fetchActions.js
index 6c4696036bb..7953ce88da2 100644
--- a/src/message/fetchActions.js
+++ b/src/message/fetchActions.js
@@ -62,8 +62,8 @@ const messageFetchComplete = (args: {|
   anchor: number,
   numBefore: number,
   numAfter: number,
-  foundNewest?: boolean,
-  foundOldest?: boolean,
+  foundNewest: boolean,
+  foundOldest: boolean,
   ownUserId: UserId,
 |}): Action => {
   const {
diff --git a/src/notification/extract.js b/src/notification/extract.js
index 772310b6e8c..cca255f0496 100644
--- a/src/notification/extract.js
+++ b/src/notification/extract.js
@@ -31,7 +31,8 @@ const asDict = (obj: JSONableInput | void): JSONableInputDict | void => {
 
     The original payload was merely `{ message_ids: [number] }`, but this has
     been expanded incrementally over the years. As of 2020-02, commit
-    2.2-dev-775-g10e7e15088, the current form of APNs messages is as follows:
+    2.2-dev-775-g10e7e15088 (released with 3.0, as "2.2" became 3.0), the
+    current form of APNs messages is as follows:
 
     ```
     type StreamData = {
@@ -82,6 +83,10 @@ const asDict = (obj: JSONableInput | void): JSONableInputDict | void => {
     ... which still didn't permit differentiating between multiple accounts on
     the same realm. This was only made possible by the addition of the `user_id`
     field, in 2.1-dev-540-g447a517e6f.
+
+    TODO(server-1.8): Simplify this comment a lot.
+    TODO(server-1.9): Simplify further.
+    TODO(server-2.1): Simplify further.
 */
 
 /** Local error type. */
diff --git a/src/notification/index.js b/src/notification/index.js
index bc26a853786..44bc12c14fc 100644
--- a/src/notification/index.js
+++ b/src/notification/index.js
@@ -37,6 +37,7 @@ export const getAccountFromNotificationData = (
   if (realm_uri == null) {
     // Old server, no realm info included.  If needed to cater to 1.8.x
     // servers, could try to guess using serverHost; for now, don't.
+    // TODO(server-1.8): Simplify this comment.
     logging.warn('notification missing field: realm_uri');
     return null;
   }
diff --git a/src/pm-conversations/pmConversationsSelectors.js b/src/pm-conversations/pmConversationsSelectors.js
index 2116debb523..37d040436c0 100644
--- a/src/pm-conversations/pmConversationsSelectors.js
+++ b/src/pm-conversations/pmConversationsSelectors.js
@@ -122,7 +122,7 @@ function getRecentConversationsModernImpl(
 
 const getServerIsOld: Selector<boolean> = createSelector(
   getServerVersion,
-  version => !(version && version.isAtLeast(model.MIN_RECENTPMS_SERVER_VERSION)),
+  version => !version.isAtLeast(model.MIN_RECENTPMS_SERVER_VERSION),
 );
 
 /**
diff --git a/src/start/AuthScreen.js b/src/start/AuthScreen.js
index d88216d37d5..aa869e3759d 100644
--- a/src/start/AuthScreen.js
+++ b/src/start/AuthScreen.js
@@ -85,6 +85,7 @@ const availableDirectMethods: AuthenticationMethodDetails[] = [
 // Methods that are covered in external_authentication_methods by servers
 // which have that key (Zulip Server v2.1+).  We refer to this array for
 // servers that don't.
+// TODO(server-2.1): Simplify this away.
 const availableExternalMethods: AuthenticationMethodDetails[] = [
   {
     name: 'google',
diff --git a/src/start/__tests__/AuthScreen-test.js b/src/start/__tests__/AuthScreen-test.js
index 283f8c608e9..42e30751795 100644
--- a/src/start/__tests__/AuthScreen-test.js
+++ b/src/start/__tests__/AuthScreen-test.js
@@ -51,6 +51,7 @@ describe('activeAuthentications: external_authentication_methods (server v2.1+ A
   });
 });
 
+// TODO(server-2.1): Delete this (and the logic it tests.)
 describe('activeAuthentications: old server API', () => {
   test('empty auth methods object result in no available authentications', () => {
     const authenticationMethods = {};
diff --git a/src/streams/streamsActions.js b/src/streams/streamsActions.js
index 54205eb53a9..e4275f0ce20 100644
--- a/src/streams/streamsActions.js
+++ b/src/streams/streamsActions.js
@@ -24,6 +24,7 @@ export const updateExistingStream = (
     //   https://github.com/zulip/zulip-mobile/pull/4748#issuecomment-852254404
     //   https://github.com/zulip/zulip-mobile/issues/4747#issuecomment-946362729
     // TODO(#4659): Ideally this belongs inside `api.updateStream`.
+    // TODO(server-4.0): Simplify this (if it hasn't already moved.)
     getZulipFeatureLevel(state) >= 64 ? value : JSON.stringify(value);
 
   const auth = getAuth(state);
diff --git a/src/users/usersActions.js b/src/users/usersActions.js
index 39e679daae1..cc813afd6f6 100644
--- a/src/users/usersActions.js
+++ b/src/users/usersActions.js
@@ -7,7 +7,6 @@ import { PRESENCE_RESPONSE } from '../actionConstants';
 import { getAuth, getServerVersion } from '../selectors';
 import { isPmNarrow, userIdsOfPmNarrow } from '../utils/narrow';
 import { getUserForId } from './userSelectors';
-import { ZulipVersion } from '../utils/zulipVersion';
 
 export const reportPresence = (isActive: boolean): ThunkAction<Promise<void>> => async (
   dispatch,
@@ -29,13 +28,12 @@ export const reportPresence = (isActive: boolean): ThunkAction<Promise<void>> =>
 // to refer to this account regardless of what the then-active account might be.
 const typingWorker = (state: PerAccountState) => {
   const auth: Auth = getAuth(state);
-  const serverVersion: ZulipVersion | null = getServerVersion(state);
 
   // User ID arrays are only supported in server versions >= 2.0.0-rc1
   // (zulip/zulip@2f634f8c0). For versions before this, email arrays
-  // are used. If current server version is undetermined, user ID
-  // arrays are optimistically used.
-  const useEmailArrays = !!serverVersion && !serverVersion.isAtLeast('2.0.0-rc1');
+  // are used.
+  // TODO(server-2.0): Simplify this away.
+  const useEmailArrays = !getServerVersion(state).isAtLeast('2.0.0-rc1');
 
   const getRecipients = user_ids_array => {
     if (useEmailArrays) {
diff --git a/src/utils/avatar.js b/src/utils/avatar.js
index afed8cd48bc..29c5c6f121b 100644
--- a/src/utils/avatar.js
+++ b/src/utils/avatar.js
@@ -46,6 +46,9 @@ export class AvatarURL {
       // this case isn't thoroughly considered, but at worst, it means a
       // 404. We could plumb through the server version and
       // conditionalize on that.
+      //
+      // TODO(server-1.9): Simplify this comment.
+      // TODO(server-3.0): Simplify this comment.
       return FallbackAvatarURL.validateAndConstructInstance({ realm, userId });
     } else if (rawAvatarUrl === null) {
       // If we announce `client_gravatar`, which we do, `rawAvatarUrl`
diff --git a/src/webview/static/base.css b/src/webview/static/base.css
index cfff2d826b0..49646ba032f 100644
--- a/src/webview/static/base.css
+++ b/src/webview/static/base.css
@@ -565,7 +565,8 @@ code, pre {
 }
 pre code {
   /* Starting with Zulip Server 3.0, code blocks have `pre > code`.
-     Undo the stuff we have for `code` that isn't meant to apply there. */
+     Undo the stuff we have for `code` that isn't meant to apply there.
+     TODO(server-3.0): Simplify this. */
   font-size: inherit;
   white-space: inherit;
   padding: 0;