NIP-09 allows notes to be deleted… it’s just that many clients don’t surface that option.
Discussion
that is just a signaling mechanism to relays and clients
primal for instance does not show notes that have "deletion requests"
there is no way to stop someone from saving a cryptographically verifiable copy of nostr posts
Agreed, nothing stops a deletion request being ignored just like nothing stops a screenshot on X.
Except, as you say, notes are cryptography verifiable to a pubkey forever.