Enable cleaning up extremities with dummy events by default to prevent undue build up of forward extremities. (#5884)

changelog.d/5884.feature

@@ -0,0 +1 @@
+Enable cleaning up extremities with dummy events by default to prevent undue build up of forward extremities.

synapse/config/server.py

@@ -355,10 +355,8 @@ class ServerConfig(Config):
 
         _check_resource_config(self.listeners)
 
-        # An experimental option to try and periodically clean up extremities
-        # by sending dummy events.
         self.cleanup_extremities_with_dummy_events = config.get(
-            "cleanup_extremities_with_dummy_events", False
+            "cleanup_extremities_with_dummy_events", True
         )
 
     def has_tls_listener(self):