GitHub.com still doesn't support IPv6. I know that there is some work going on to support it, but this shows that it is far from trivial.
They also did offer IPv6 availability for a short while as a test, but that was quickly shut down, so there is probably a technical issues they can't figure out or they would've kept the trial system running for longer.
Either way, Github isn't communicating, so it's hard to tell if this is indifference or incompetence. As an end user, the distinction doesn't really matter.