It goes on to say "Simply visit the GitHub Copilot subscription page to see if you are one of the open source maintainers that meet our criteria for a complimentary subscription"

When I go to https://github.com/github-copilot/free_signup it says:

" Congratulations! You are eligible to use GitHub Copilot for free.

Thanks for being a part of our open source and education communities. GitHub Copilot uses the Codex AI model to offer coding suggestions."

I have a project with about 3k stars, and regularly contribute to another project ~4k stars (Where I'm also the primary maintainer, although it's not on my account), as well as some things in with hundred and dozens of stars.

I don't how high up that is in the ranking, although given that most projects get 0 stars I suspect it's probably higher than you'd might expect.