Or get the source code using Git, which actually (by design) guarantees that its checksums are stable.

Downloading a source tarball is significantly cheaper on both sides than git. A source tarball is 100% served from CDNs, whereas I don't believe the same isn't quite true for git (even over https).

That's a good point.

It's way more resource-intensive and much slower, which is why it's not preferred in Nixpkgs, for example.

But it's also vulnerable to the same problem in that your package Manager's build system is still dependent on GitHub. It will take more to screw you up, but a whole GitHub outage, for example, will definitely still hurt.

It's not depending on github-specific functionality tho. You can just redirect it to another mirror of git repo of the project