https://github.com/nostr-protocol/nips/blob/master/40.md
idk what relays support this... i have an event store that has garbage collection, probably would make sense for it to delete those events at some time not long after the expiry
or maybe it could be implemented by creating a "todo" list with the event ID and a timestamp that gets checked more regularly than the GC (which would be like 10 minutes)
also, the other thing you'll encounter is clients will cache the event indefinitely as well
afaik this nip isn't widely supported