> For a different source, although not representative of all code, the repositories for a Linux distro like Debian or Arch would give interesting data.
Source code from Linux distro repos should be different but far from representative or meaningful, specially as it leans heavily towards legacy C libraries and tooling. I hardly see how that could be a meaningful rejection of GitHub as a statistically significant sample of what real world projects are actively being developed by real world developers.
For a different source, although not representative of all code, the repositories for a Linux distro like Debian or Arch would give interesting data.