There is no such thing.
Centralized platforms give us the illusion of being able to delete our data, but we have no way of knowing whether they or anyone else with an interest in doing so, has created a copy.
It is also impossible to prove the non-existence of anything.
Deletion on Nostr will always be "best effort," and a client like Amethyst in particular really can't do anything about that, because deletion policy is handled where the data is stored: on the relays. All Amethyst can do, which I believe it already does, is send a deletion request to all known relays in accord with NIP-62. It is then up to that relay to comply with that request.
Relays that are offline, or that are not public may never receive that request. Still other relays might receive the request and ignore it. There is nothing Amethyst or any other client can do to prevent that.