😢 @aaronpk knows my stance on this well - domain-based auth is exclusionary and confusing to users. IndieAuth should just use email addresses, even if it's not doesn't use webfinger and just does s/@([^.*]\..*$/\1/ with the address.
There is no obligation that you have to register your own domain for IndieAuth to work. I’ve talked about this at ActivityPub Conference showing how they can use IndieAuth to enable a standards-based app ecosystem for ActivityPub/Mastodon apps. That of course uses shared domains.