I also noticed that relays in general have no way to inform for how long should clients backoff.
I've just submitted a PR to address that problem: https://github.com/nostr-protocol/nips/pull/2498
Pinging some relay owners fiatjaf (npub180c…h6w6) someone (npub1nlk…jm9c) jb55 (npub1xts…kk5s) semisol (npub1226…grkj) Cameri (npub1qqq…n47m). Please review the PR.