I’ve just noticed that it doesn’t work at all to use XObjects to display a notification filter: While the wiki components are correctly registered, the code that should load them DefaultNotificationFilterManager uses a standard component manager to look for them and thus doesn’t find any components that are registered at the wiki level and thus no wiki components at all. The same will be true for any extensions installed at wiki level I think. Therefore, it might even be okay to just delete at least parts of that code without causing any breakage as the code is already broken.
+1 to deprecate or remove those APIs.