Idk about nostr, but yes, the decentralized nature of the fediverse makes this very hard/impossible. Who has the authority on what’s your login? What if I don’t want a user to login on my instance because they’re a known spammer? Etc

Not sure it’s a fitting term. Most instances aren’t communities. They provide their service, but there’s no “we are the members of mastodon.community and here’s what we do as a community”. They exist but aren’t the norm. Calling instances communities probably leads to wrong expectations

KYC = know your customer

For everyone else but knowing