Try the following:
$ nslookup github.com
[...]
Non-authoritative answer:
Name: github.com
Address: 140.82.121.3
See also the completely ignored post in their forums.
I used to study networking, albeit at a pretty beginner level. IPv6 has been around for nearly 30 years at this point, so I’d be surprised if the hardware github uses doesn’t support it. The impression I got was that it’s pretty easy to extend an IPv4 address space so there isn’t any rush to make a large scale move to IPv6 everywhere.