One pretty huge downside for GitHub Pages, is that they still don't seem to serve content to IPv6 addresses.

Saying that because I tried it out again a few weeks ago for a friend's website (https://neasair.org), and nope... it doesn't provide AAAA DNS records (IPv6), just A (IPv4).

So, had to put the website on some other infrastructure already hanging around, that does do AAAA records.