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.

Outdated beliefs probably.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 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.

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.