Clients not pulling delete notes is a bug imo but I think your point still stands. If you request your subscription relays to delete your note, best case is that they delete it from their own databases. There could still be backups of that note somewhere, in caches or in other peoples backups which means that the event isn't deleted from the world at all(as I expect most people to think note deletion is). The note can still be shared around out-of-band, we can still see the contents and we can still verify the signature.