The formal spec may have been removed amid edits and shuffling. Some event kinds have specific d tag usage defined, but there's no formal standard for all events, that I could find.
Discussion
I don't know, I'm thinking it's a short uuid,
but in wiki format it's like the title https://github.com/nostr-protocol/nips/blob/master/54.md
The wiki spec uses a title-like format because it can be used to build human-readable URLs within the client.
Also never found one. I've read all of the NIPs, multiple times. Don't recall seeing one.