github.com doesn't have a AAAA DNS entry. So it's not serving anything directly over IPv6. Likewise, ping -6 github.com fails. So, what are you seeing that is supporting ipv6?
That is interesting. I figured they would be something like cloudflare/other redirection for github pages. But the IPv6 address space is github registered.
So, really not sure why they don't have the rest of their site enabled.