diff --git a/src/NHibernate/Async/Engine/ForeignKeys.cs b/src/NHibernate/Async/Engine/ForeignKeys.cs
index 8185a996af..4e91d411bd 100644
--- a/src/NHibernate/Async/Engine/ForeignKeys.cs
+++ b/src/NHibernate/Async/Engine/ForeignKeys.cs
@@ -8,6 +8,7 @@
 //------------------------------------------------------------------------------
 
 
+using System;
 using NHibernate.Id;
 using NHibernate.Persister.Entity;
 using NHibernate.Proxy;
@@ -152,6 +153,8 @@ private async Task<bool> IsNullifiableAsync(string entityName, object obj, Cance
 		/// <remarks>
 		/// Hit the database to make the determination.
 		/// </remarks>
+		// Since v5.6
+		[Obsolete("This method has no more usages and will be removed in a future version")]
 		public static async Task<bool> IsNotTransientSlowAsync(string entityName, object entity, ISessionImplementor session, CancellationToken cancellationToken)
 		{
 			cancellationToken.ThrowIfCancellationRequested();
diff --git a/src/NHibernate/Engine/ForeignKeys.cs b/src/NHibernate/Engine/ForeignKeys.cs
index fe8dbb9cde..ed2e7c404d 100644
--- a/src/NHibernate/Engine/ForeignKeys.cs
+++ b/src/NHibernate/Engine/ForeignKeys.cs
@@ -1,3 +1,4 @@
+using System;
 using NHibernate.Id;
 using NHibernate.Persister.Entity;
 using NHibernate.Proxy;
@@ -151,6 +152,8 @@ private bool IsNullifiable(string entityName, object obj)
 		/// <remarks>
 		/// Hit the database to make the determination.
 		/// </remarks>
+		// Since v5.6
+		[Obsolete("This method has no more usages and will be removed in a future version")]
 		public static bool IsNotTransientSlow(string entityName, object entity, ISessionImplementor session)
 		{
 			return !IsTransientSlow(entityName, entity, session);