Perhaps a little tin foil hatty and definitely not the only reason but Microsoft owns Github and also makes a boatload of money off of Azure. Incumbent cloud providers like Azure have a major advantage in terms of having plenty of IPv4 addressing available whereas a new entrant to that market would have to buy or lease that space at a premium. Thus, these companies have an incentive to keep IPv4 a necessity.

IPv6 is very difficult to implement and enforce reliable rate limits on anonymous traffic. This is something we've struggled a lot with - there is no consistent implementation or standard when it comes to assigning of IPv6 addresses. sometimes a machine gets a full /64, other times a whole data center uses a full /64. So then we need to try and build knowledge of what level to block based on which IP range and for some it's just not worth the hassle.

Outdated beliefs probably. When I talk about v6 support in our b2b saas, PM laughs and says nobody uses that shit. Big tech are massive laggards on this funnily enough.

It's a possibly a managerial thing, which KPI are you improving when spending engineering time on adding IPv6 support?

That said, for their HTTP stack they use fastly (as far as I understand), which should make the shift moderately easier.

IPv6 rollout is a lot of operational work that ends with next to no immediate quantifiable benefit. So I’ll never be prioritized in a cost-cutting environment.

It could be that they don't want to implement IP bans in IPv6.

You probably need a hefty security reimplementation if you want to add IPv6 to Github.