This has always been a weird quirk of GitHub. You can disable issues, boards, wikis, but pull requests cannot be toggled. It's a pain point for lots of projects: those closed to contributions, those which are not primarily code (issues-only), those that use a different platform for review (e.g. Gerrit), ...

Why does GitHub persist? It's not like forcing the feature enabled helps anyone. The maintainers will still not merge if they don't want to. There are bots in the marketplace that will close PRs with a message. GitLab has a toggle. Why is this so important to GitHub?