Hi!
In 2018 Github Pages announced support for custom domains with HTTPS: https://github.blog/2018-05-01-github-pages-custom-domains-https/
This meant that Github had to generate certs which pointed to the custom domains the users were configuring for their pages. This worked perfectly until a few weeks ago, when all certs started to point to the default domains instead of the custom domains.
That bug breaks all the Github Pages that were using custom domains and HTTPS, because most browsers (Chrome, Firefox, Safari) now block access via HTTPS, warning the users that the sites are insecure and could steal info, etc.