It might be possible for message retention policies to break sticky event catchup transactions by deleting the prev_event #20357

When sending an event over Federation /send, the remote homeserver (for Synapse at least) requires you to be able to supply its prev_events on demand, over /get_missing_events.

When sending old MSC4354 sticky events (#19409) in the room to a remote server, it feels like it might be possible for the prev_event of the sticky event to have been deleted, which would cause that federation transaction to fail.

I intend to confirm if this is an issue or not, with a test.