This NIP is not yet in main. Here is an overview of PRs referencing this NIP. 
Clarify NIP-42 flow OPEN @arthurfranca
Add AUTH third element CLOSED @arthurfranca
Implement Markdownlint GitHub action OPEN @mariano-perez-rodriguez
Markdown improvements, embelishments, and standardizations CLOSED @mariano-perez-rodriguez
Update all NIPs to include 'depends' and 'mentions' tags OPEN @blakejakopovic
add nip-42: authentication MERGED @fiatjaf
NIP-42 - External Embed Sizes CLOSED @deanpress