This NIP is not yet in main. Here is an overview of PRs referencing this NIP. 
Reword set_metadata to metadata MERGED @AsaiToshiya
NIP-89: Recommended Application Handlers MERGED @pablof7z
NIP-89 Ephemeral Bitcoin Transaction Package Relay OPEN @benthecarman
NIP-89: payto: Payment Targets OPEN @ATXMJ