Look at how it shat on Markdown with what it calls "GitHub Flavored Markdown". Look at the things that it calls "wikis". Look at how GitHub's PR merge tool junks up the commit log. Look at how many projects don't even have a way to accept a fix unless you submit it with GitHub's janky pull request workflow. Hell, a bug in Netlify's command-line client managed to make its way into release versions that would straight up cause the process to terminate when cwd was a repo that wasn't hosted on github.com, leading to unhandled exception.
The tacit assumption that you're using GitHub is like the tacit assumption 15 years ago that you were using Visual Studio, only this time, you can't escape just by steering clear of Windows-related tech.