Seems like GitHub could solve this by making users verify they own a domain name by adding a value to a txt record rather than just seeing the domain points to github and letting any repo use it.
I’ve set up two static pages with custom domains using GH Pages in the past couple of months, and both times I had to go digging in the docs before I found the verification page as part of trying to figure out why https wasn’t working. Fucking inexplicably poor UX design from GH. If I add a custom domain, just ask me to verify it.