People can just block all the relays that don't respect this. It's similar to how virtually every relay accepts event deletions.
Discussion
Have any relays (and clients) implemented it already?
yes. its supported on immortal: https://github.com/dezh-tech/immortal
We support it in nostr:npub1pu3vqm4vzqpxsnhuc684dp2qaq6z69sf65yte4p39spcucv5lzmqswtfch!
Rebroadcasting seems to be the elephant in the room... Clients of readers can push your notes to relays you don't want.
what about posts that were already used to train ai?