Hosting private repositories on GitHub pages is another feature only available with the Pro plan. Figured this one out a few days after canceling my subscription.
I figured it out on the day my subscription lapsed, and someone created their own repository using my domain in their CNAME file. I had to wait until GitHub banned the user until I could set up the domain with GitHub Pages again.
It’s free on gitlab.com and you can have as many as you want. I recently parked about 10 domains there.