Remove `account_threepid_delegates` option

[anoadragon453]

With #5835, we're allowing homeservers to be able to send their own registration emails. Earlier, we allowed homeservers to be able to send password reset emails themselves. Both of these options were just considered temporary to migrate people over from relying on vector.im or another identity server to handle SMTP, to handling SMTP themselves if they required the functionality.

Once a sufficient amount of time has passed, we should remove these options and yell from the mountains that people need to manage SMTP from their Synapse install or disable it altogether.

[erikjohnston]

Removing trust_identity_server_for_password_resets seems fine, but I'm less sure about account_threepid_delegate. We don't seem to mention in the sample config that using it is delegated (its just in the UPGRADE notes), and is also currently the only way of support validating phone numbers?

[richvdh]

sigh. That's annoying, as it makes #9677 harder.

[erikjohnston]

We should figure out if MSISDN is still a thing that is a) used and b) we want to continue to support. It would be nice to strip all this out if its not being used. We should pull the numbers out for how often msisdn is used in Sydent.

Otherwise, we need to port the code to use the new v2 sydent API (or make this work with v2 API)

[anoadragon453]

We've identified one Element customer that still makes use of this option, though has not had a msisdn registration on their identity server since March 2021. So this may no longer be a concern for any known entity.

However, the context for why some wish to make use of this feature is to improve UX when creating mappings on an identity server for every user is a necessity - rather than something that can happen lazily and at the user's request.

Essentially, if you need 3PID<->MXID mappings to appear on your identity server during user registration (without asking the user to validate their 3PID twice - once for the hs and another time for the is), this feature - plus some Sydent modifications - is one way of going about it.

DINUM were using account_threepid_delegates (for delegation of email verification), but are no longer. Their need was to have the identity server be aware of user's emails - but this has been sidestepped by a little hack to instead use Sydent's internal bind API to register 3PID mappings. They also do not validate msisdns.

Of course it's also nice to allow users to add their phone numbers to their homeserver account. And in Element's UI today, you can't add a phone number to an identity server without first adding it to your homeserver. TravisR has also helpfully noted that Element Web does still support msisdn verification, but matrix.org does not present the UIAA stage as an option to trigger it.

Removing trust_identity_server_for_password_resets seems fine

I'm not aware of anyone else using trust_identity_server_for_password_resets either.