[ollemasle]

They just posted this update on GitHub Status - related to GitHub Pages:

  We are aware of TLS problems related to an expired certificate in our CDN configuration.
  We are working to resolve this issue.

See https://www.githubstatus.com/incidents/x7njwb481j9b

[ollemasle]

And now (Mar 24, 2023 - 21:10 UTC):

   This incident has been resolved.

This is indeed resolved.