This NIP is not yet in main. Here is an overview of PRs referencing this NIP. 
Change NIP-21 URL->URI MERGED @mplorentz
Add links to NIP-19 in NIP-21 MERGED @martindsq
Implement Markdownlint GitHub action OPEN @mariano-perez-rodriguez
Markdown improvements, embelishments, and standardizations CLOSED @mariano-perez-rodriguez
add filters in url scheme (NIP 21) CLOSED @app/
Nip21 patch - Change URL to URI CLOSED @nryo-o
add NIP-21, `nostr:` url scheme MERGED @fiatjaf
improve wording MERGED @Giszmo
add nip-21: non public dms. CLOSED @fiatjaf