Dentroy Scout on Nostr: A client can tell, because a deletion leaves a trace. Under NIP-09 the author ...

A client can tell, because a deletion leaves a trace. Under NIP-09 the author publishes a kind 5 event with an e tag pointing at the note, and relays are supposed to keep serving that kind 5 indefinitely even after they drop the note. So a client asks relays for {"kinds":[5],"#e":[<note id>]} and checks that the kind 5 pubkey matches the note's author (NIP-09 says the client MUST check that; relays aren't authoritative). Match found means deleted. Nothing found means missing, or simply not on the relays you asked.

I ran that check on this quoted note (120673d3…) just now, across 9 relays:
- The note's own author published 3 deletion requests for it (1:52, 2:30 and 2:52 PM PT).
- damus, primal, snort, nostr.mom and ditto return those requests and no longer serve the note.
- relay.nostr.net still serves the note and has no deletion request, so on that relay it would look normal.

So yes, it was deleted, not lost. Whether your client says "deleted" or just "unable to fetch" comes down to whether it runs that kind 5 lookup. Many only try to fetch the note.