Refs #27236 -- Doc'd that AlterIndexTogether is no longer officially supported for Django 4.2+ migration files.

docs/ref/migration-operations.txt

@@ -102,9 +102,17 @@ subclass).
 
 .. class:: AlterIndexTogether(name, index_together)
 
-Changes the model's set of custom indexes (the
-:attr:`~django.db.models.Options.index_together` option on the ``Meta``
-subclass).
+Changes the model's set of custom indexes (the ``index_together`` option on the
+``Meta`` subclass).
+
+.. warning::
+
+    ``AlterIndexTogether`` is officially supported only for pre-Django 4.2
+    migration files. For backward compatibility reasons, it's still part of the
+    public API, and there's no plan to deprecate or remove it, but it should
+    not be used for new migrations. Use
+    :class:`~django.db.migrations.operations.AddIndex` and
+    :class:`~django.db.migrations.operations.RemoveIndex` operations instead.
 
 ``AlterOrderWithRespectTo``
 ---------------------------

docs/releases/4.2.txt

@@ -310,6 +310,13 @@ Running the :djadmin:`makemigrations` command will generate a migration
 containing a :class:`~django.db.migrations.operations.RenameIndex` operation
 which will rename the existing index.
 
+The ``AlterIndexTogether`` migration operation is now officially supported only
+for pre-Django 4.2 migration files. For backward compatibility reasons, it's
+still part of the public API, and there's no plan to deprecate or remove it,
+but it should not be used for new migrations. Use
+:class:`~django.db.migrations.operations.AddIndex` and
+:class:`~django.db.migrations.operations.RemoveIndex` operations instead.
+
 Miscellaneous
 -------------