As discovered by @tulir, when using the MSC2716 /batch_send endpoint, events specified in state_events_at_start will come down incremental /sync and the application services /transactions endpoints. Since this is historical state, it shouldn't be coming down the live pipe.

We already have a Complement tests to ensure the historical messages don't come down incremental sync, but we should also do the same for the state events.

Relevant code

• Where we create state events from the state_events_at_start: synapse/handlers/room_batch.py#L213-L249
• builder.internal_metadata.historical = True: synapse/handlers/message.py#L608
• If it's a historical event, then we mark it as backfilled = True: synapse/handlers/message.py#L1526-L1530