Another way to frame this is that even though the FFmpeg repository clearly indicates it's a mirror repository and has had a single open pull request titled "WARNING: PULL REQUESTS ON THIS REPOSITORY ARE IGNORED" [0] sitting in the pull requests page for 10 years, they've still had to close hundreds of pull requests.
That’s fair, but it would be trivial to use GitHub’s actions/API (and probably just an existing open source tool in either case) to automatically close them without anyone wasting their time.
I think you’re definitely right that being a massive project on GitHub that accepts PRs there would be a nightmare though.
