So I can no longer log into my GitHub account and tried all the recovery steps. Sadly, even the list of 10-digit recovery codes don't seem to work. Did I run afoul of some community standards? The most controversial thing I have hosted on GitHub is https://ohmeadhbh.github.io/bobcat/ - maybe the powers that be don't like my low-styling / no-javascript proclivities?
Or more likely it's some weird corner case. I was a little uncomfortable when MSFT bought GitHub, but it was reasonably easy to use, so I kept with it. So unless this is a known problem, I'm back to self-hosting and/or GitLab.