The user directory background tasks are controlled by a separate flag.

docs/workers.md

@@ -318,6 +318,9 @@ the shared configuration would include:
 run_background_tasks_on: background_worker
 ```
 
+You might also wish to investigate the `update_user_directory` and
+`media_instance_running_background_jobs` settings.
+
 ### `synapse.app.pusher`
 
 Handles sending push notifications to sygnal and email. Doesn't handle any

synapse/handlers/user_directory.py

@@ -54,7 +54,7 @@ class UserDirectoryHandler(StateDeltasHandler):
         # Guard to ensure we only process deltas one at a time
         self._is_processing = False
 
-        if hs.config.run_background_tasks and self.update_user_directory:
+        if self.update_user_directory:
             self.notifier.add_replication_callback(self.notify_new_event)
 
             # We kick this off so that we don't have to wait for a change before

synapse/server.py

@@ -188,7 +188,6 @@ class HomeServer(metaclass=abc.ABCMeta):
     REQUIRED_ON_BACKGROUND_TASK_STARTUP = [
         "auth",
         "stats",
-        "user_directory",
     ]
 
     # This is overridden in derived application classes
@@ -255,6 +254,9 @@ class HomeServer(metaclass=abc.ABCMeta):
         self.datastores = Databases(self.DATASTORE_CLASS, self)
         logger.info("Finished setting up.")
 
+        # Register background tasks required by this server. This must be done
+        # somewhat manually due to the background tasks not being registered
+        # unless handlers are instantiated.
         if self.config.run_background_tasks:
             self.setup_background_tasks()