Since we launched this last year, GitHub released a v2 of their internal cache API [0], based on Twirp [1] of all things, so we adapted to that. Interestingly that Twirp service also receives Actions artifacts, though we have not intercepted those today given that you likely still want them to appear in the GitHub UI / be accessible from the GitHub API.
[0] https://github.com/actions/cache/discussions/1510
[1] https://github.com/twitchtv/twirp