Don't delete OFFLINE users from the presence cache, so we can report on their offline mtime

synapse/handlers/presence.py

@@ -231,9 +231,6 @@ class PresenceHandler(BaseHandler):
         #   we don't have to do this all the time
         self.changed_presencelike_data(target_user, state)
 
-        if not now_online:
-            del self._user_cachemap[target_user]
-
     def changed_presencelike_data(self, user, state):
         statuscache = self._get_or_make_usercache(user)