Just as a clarification, much of what you mention has only changed in the last year or so. For a very long time gh-pages branch and Jekyll were assumed (you couldn't even use plugins with Jekyll). Custom domains weren't free. SSL wasn't free. Github is doing great stuff but it has taken time to evolve into the all free all you can eat buffet you see today.
Get your facts straight, they have had custom domains for a looong time & there was absolutely no need for Jekyll either at any point to host a page, it just happened to be one the most documented/blogged about way to use GitHub pages with automatic builds instead of having static content.