PRs are not optional: there is no way to disable them on GitHub. I can't be sure that this is intentional, but it certainly works out well for them that this is one of many properties which make it quite difficult to migrate away from the platform.

PRs aren't an optional feature, though acting on PRs is obviously optional; nothing prevents you from ignoring or (even automatically) closing all PRs from anyone who is not on a list of approved contributors.

Pull requests are not optional on GitHub. Users have been begging for more than a decade for an option to disable pull request for a repository, and GitHub continues to ignore them.