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/
Do you have an example? Because it works great on openwrt, dd-wrt, pfsense, opnsense, unifi, mikrotik....and then if you're using the isp equipment it works out of the box.