The official NIP list on Github to be precise.
Discussion
Not sure my guess is fiatjaf and some other folks but it’s not really critical because anyone can implement any nip even if it’s not in GitHub. A few clients deciding they want to use it, can just go ahead and do so.
I think it's also useful to note that there are 65535 (or 65536?) event kinds usable, so there are many possibilities using Nostr.