Deletion anywhere on the internet is an illusion. Your supposedly deleted post is hidden from your view on Facebook or X, or other platforms, but did they actually purge it from their database? Maybe.

We must always operate under the assumption that what we post on the internet is like toothpaste that can't be put back in the tube.

#Nostr is no different, it's just honest about it. You can "request deletion" on some clients, but the relays your note has already propagated to may or may not comply or even receive the request.