It’s a fully standardised and formally spec’d protocol!
https://github.com/concord-protocol/concordSince Concord is very large/broad functionally, it has its own proposal system (CORDs rather than NIPs), it wouldn’t be feasible to condense it to a single “NIP”, but it is very much a growing standard in Nostr with over 6-7+ clients using it. 🙏💚