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/
This is less to do with the ISPs and more to do with the implementation of DHCP-PD renewals on various software/hardware devices. I'm not going to point any fingers, but it seems that some vendors don't play very nicely with other vendors.