You presume incorrectly. GitHub Packages, the package registry and binary hosting service, does not even support Nix. As for GitHub in general—
The binary hosting at https://cache.nixos.org/ is independent of GitHub, and so are the old-style channels at https://channels.nixos.org/. The new-style flake registry used to be fetched from GitHub but has now been moved to https://channels.nixos.org/flake-registry.json. Admittedly in a new-style situation you’re likely to be using unlocked flake references that refer to GitHub (e.g. Nixpkgs), but it’s on you to lock them and pull them into your Nix store in that case.
Of course, you also get GitHub references for upstreams that host their code there, but that applies to almost any distro(’s build system) except the oldest of the old-timers which host the source for the whole distro on their own infrastructure, like Debian. (I happen think the old-timers are right here, but that’s beside the point.)
This is actually the second (maybe third; I didn't even know about GitHub's outage a week and a half ago so idk how Nix was or was not affected) time in three months or less that a partial GitHub outage or GitHub change has taken down Homebrew while leaving Nix unaffected.
