Context:
People have been asking for IPv6 Support on GitHub since years (probably a decade by now)
... and someone even got so annoyed that they decided to setup a dedicated website for checking this: https://isgithubipv6.live/
Adding warnings will only result in annoying people, not driving for improvement.
Given how poorly adoption has gone so far this might be the only way to get actual fast support rolled out. Piss people off, get change