diff --git a/synapse/handlers/sliding_sync.py b/synapse/handlers/sliding_sync.py
index 5c67b19a78..6e4ac23a87 100644
--- a/synapse/handlers/sliding_sync.py
+++ b/synapse/handlers/sliding_sync.py
@@ -336,6 +336,9 @@ class SlidingSyncHandler:
                 # Apply filters
                 filtered_room_ids = room_id_set
                 if list_config.filters is not None:
+                    # TODO: To be absolutely correct, this could also take into account
+                    # from/to tokens but some of the streams don't support looking back
+                    # in time (like global account_data).
                     filtered_room_ids = await self.filter_rooms(
                         sync_config.user, room_id_set, list_config.filters
                     )