Reinstate removed EventBase methods (#19712)

Both `__getitem__` and `.user_id` were removed in #19680 to simplify the
event class. However, `EventBase` is exposed to modules who might also
make use of those methods, so let's reinstate them (but otherwise not
reinstate the usage of them in the code).

tests/module_api/test_api.py

@@ -828,6 +828,24 @@ class ModuleApiTestCase(BaseModuleApiTestCase):
         # Ensure the pushers were deleted after the callback.
         self.assertEqual(len(self.hs.get_pusherpool().pushers[user_id].values()), 0)
 
+    def test_event_deprecated_methods(self) -> None:
+        """Test that deprecated methods on events are still functional."""
+        user_id = self.register_user("user", "password")
+        tok = self.login("user", "password")
+
+        room_id = self.helper.create_room_as(tok=tok)
+
+        state = self.get_success(
+            self.hs.get_storage_controllers().state.get_current_state(room_id)
+        )
+        create_event = state[(EventTypes.Create, "")]
+
+        # `.user_id` is a deprecated alias for `.sender`.
+        self.assertEqual(create_event.user_id, user_id)
+
+        # The event supports looking up keys via `__getitem__` although deprecated
+        self.assertEqual(create_event["room_id"], room_id)
+
 
 class ModuleApiWorkerTestCase(BaseModuleApiTestCase, BaseMultiWorkerStreamTestCase):
     """For testing ModuleApi functionality in a multi-worker setup"""