Ephemeral posts are posts that only exist for a while and are automatically deleted after.

I'd suggest to use two mechanisms simultaneously.

1. Make the originating server send a Delete message once the expiration date is reached
2. Add an expires date field to the Note object itself, analog to published. Servers who know about this should periodically delete all notes which have an expires data that's in the past.

This could still miss some deletions (like relayed public posts) but should catch the majority of cases.