When people talk about just "OpenID", they usually mean the original one, not Connect. The old OpenID did not have the ability to authenticate requests to the domain you signed in with.
OpenID Connect adds lots of bloat, it's very large and complicated. IndieAuth is actually easy to understand and implement.
Depends on your point of view. IndieAuth has a bunch of modifications to OAuth2 behaviors that may not be compatible with existing deployments, and requires software to implement things like full HTML parsing libraries to read out link tags. There were many incomplete OpenID 1/2 implementations, and compatibility greatly suffered as a result.
As someone who maintained an OpenID 1/2 OP for a few years, I would much rather implement OpenID Connect Basic Profile than IndieAuth.
IndieAuth, like OpenID 1/2, also assumes the user has a known profile page or even knows what a URL is, which are both statistically unlikely.
It all mentioned
- user identity: core user info endpoint
- discovery: https://openid.net/specs/openid-connect-discovery-1_0.html
- client registration: https://openid.net/specs/openid-connect-registration-1_0.htm...
And also other features which are important for more complex cases than just simple "login using X" button.