Theoretically, Amethyst. And maybe Gossip?
Freerse has plans to implement. Same with Nostrudel.
stop me if you remember me talking about NIP-42 before
please remind me also how many clients actually support it? anyone other than nostr:npub180cvv07tjdrrgpa0j7j7tmnyl2yr6yr7l8j4s3evf6u64th6gkwsyjh6w6 and nostr:npub1jlrs53pkdfjnts29kveljul2sm0actt6n8dxrrzqcersttvcuv3qdjynqn ???
Discussion
it's just signing a special type of event and a different envelope, it's really not hard, the spec has existed, and fiatjaf's implementation has been out there for maybe 6 months now, it's just a matter of priorities... nostr:npub18kzz4lkdtc5n729kvfunxuz287uvu9f64ywhjz43ra482t2y5sks0mx5sz sorta gets it but doesn't get it that relays need to all start implementing the capability... idk if he's open sourced the strfry sprocket for it but the lack of reference implementations on the relay side is a big problem too
khatru can already do it tho...