This NIP is not yet in main. Here is an overview of PRs referencing this NIP. 
For improved SEO of web clients, add canonical tag to NIPs 23 and 52. OPEN @shawndearmond
Remove description about NIP-12 MERGED @AsaiToshiya
merge nips 12, 16, 20 and 33 into nip 01 MERGED @fiatjaf
NIP-52: Calendar Events, Calendars, Calendar Event RSVPs MERGED @tyiu
NIP-52 Badges (prev Issued Claims).md CLOSED @neilck