This is no different from "do not track" and relies on people respecting it, lol good luck
Discussion
People can just block all the relays that don't respect this. It's similar to how virtually every relay accepts event deletions.
Have any relays (and clients) implemented it already?
yes. its supported on immortal: https://github.com/dezh-tech/immortal
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?
Relays generally do not want to piss off users. It's the same reason most people search sites delete (or at least hide) your data if you ask.