NIP-39: External Identities in Profiles

Nostr protocol users may have other online identities such as usernames, profile pages, keypairs etc. they control and they may want to include this data in their profile metadata so clients can parse, validate and display this information.

This is the only true verification that we have.

IMO, more Nostr clients should support displaying NIP-39 on profile pages. At this time I believe it's only Amethyst that supports this NIP. This NIP is almost 2 months old and hasn't gained a lot of support. Why? Which other services should be added to this NIP? Keybase seems like an easy addition. Others?