github.com doesn’t have an IPv6 address.

github.io does have an IPv6 address. Indeed, one workaround for getting rate limited when using a carrier NAT with github.com is to have a github.io page and pull data from github.io instead of github.com.

Edit: About a decade ago, all of my hosting had full IPv6 support, and I tried to move over to IPv6. However, there was an issue with Letsencrypt certs not validating over IPv6, so I made my web pages IPv4 only. Recently, I gave IPv6 a go again, and the cert issue has been fixed, so now my webpages finally have both IPv4 and IPv6 addresses.