Haha, you'd think. You can disable issues, but not pull requests, which has been a pain point for the last... forever[0]. Best you can do is setup workflows to automatically close and lock every pull request (and optionally, add an exception for a single pull request that says in all caps "DO NOT OPEN PULL REQUESTS"). It's as painful as it sounds.

[0]: https://github.com/github/dmca/pulls

You can disable issues but not PRs. People faux-disable it by opening a PR with "don't open PRs" or set up a bot to comment on PRs.

You can open source something without putting it on GitHub.

You just succintly described what will kill off github eventually