Do not include rooms with an unknown room version in a sync response. (#10644)

A user will still see this room if it is in a local cache, but it will
not reappear if clearing the cache and reloading.

synapse/handlers/sync.py

@@ -1,5 +1,4 @@
-# Copyright 2015, 2016 OpenMarket Ltd
-# Copyright 2018, 2019 New Vector Ltd
+# Copyright 2015-2021 The Matrix.org Foundation C.I.C.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -31,6 +30,7 @@ from prometheus_client import Counter
 
 from synapse.api.constants import AccountDataTypes, EventTypes, Membership
 from synapse.api.filtering import FilterCollection
+from synapse.api.room_versions import KNOWN_ROOM_VERSIONS
 from synapse.events import EventBase
 from synapse.logging.context import current_context
 from synapse.logging.opentracing import SynapseTags, log_kv, set_tag, start_active_span
@@ -1843,6 +1843,9 @@ class SyncHandler:
         knocked = []
 
         for event in room_list:
+            if event.room_version_id not in KNOWN_ROOM_VERSIONS:
+                continue
+
             if event.membership == Membership.JOIN:
                 room_entries.append(
                     RoomSyncResultBuilder(