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.
These guys do an insane amount of engineering to speed up builds and everyone follows. Love their blogs.
Zero chance I'd sign up for their competitors knowing how good the product is and that everyone else follows them. I don't know why Docker, JFrog, or GitHub don't just buy them already.