@Moon even a delete log/feed that other servers refresh once every few minutes. Using something like SSE they can keep their place and resume the feed from last known good position. The feed should not keep data older than a week so any server offline for a long time may miss deletes.

This would scale 100% better and if a user has been deleted the delete is ignored. User deletes would be the only federated deletes, or maybe not at all if this log could be used?