Add m.require_identity_server to /versions unstable_flags (#5972)

Author: anoadragon453

changelog.d/5972.misc

@@ -0,0 +1 @@
+Add m.require_identity_server flag to /version's unstable_features.

synapse/rest/client/versions.py

@@ -48,7 +48,19 @@ def on_GET(self, request):
                     "r0.5.0",
                 ],
                 # as per MSC1497:
-                "unstable_features": {"m.lazy_load_members": True},
+                "unstable_features": {
+                    "m.lazy_load_members": True,
+                    # Advertise to clients that they need not include an `id_server`
+                    # parameter during registration or password reset, as Synapse now decides
+                    # itself which identity server to use (or none at all).
+                    #
+                    # This is also used by a client when they wish to bind a 3PID to their
+                    # account, but not bind it to an identity server, the endpoint for which
+                    # also requires `id_server`. If the homeserver is handling 3PID
+                    # verification itself, there is no need to ask the user for `id_server` to
+                    # be supplied.
+                    "m.require_identity_server": False,
+                },
             },
         )