Note that m.tag events can appear in v1 initialSync and /events as well as v2 sync. Only add the room_id for v1 /events since it is redundant in v1 /initialSync

api/client-server/v1/sync.yaml

@@ -248,8 +248,7 @@ paths:
                           "visibility": "private",
                           "private_user_data": [{
                             "type": "m.tag",
-                            "content": {"tags": ["work"]},
-                            "room_id": "!TmaZBKYIFrIPVGoUYp:localhost"
+                            "content": {"tags": ["work"]}
                           }]
                       }
                   ]

specification/modules/tags.rst

@@ -10,8 +10,13 @@ user that set them but are shared across all their devices.
 Events
 ------
 
-The tags on a room are passed as single ``m.tag`` event in the
-``private_user_data`` section of a room in v2 sync.
+The tags on a room are receieved as single ``m.tag`` event in the
+``private_user_data`` section of a room in a v2 /sync.
+
+The ``m.tag`` can also be received in a v1 /events response or in the
+``private_user_data`` section of a room in v1 /initialSync. ``m.tag``
+events appearing in v1 /events will have a ``room_id`` with the room
+the tags are for.
 
 {{m_tag_event}}