The project owner could of course just write “No” without any explanation, but most projects I’ve seen that don’t take PRs have a reason for it: they prefer patches by mailing list, or don’t accept contributions but have a forum where you can post patches, or are a mirror of an upstream you should contribute to instead, or whatever. Usually this information is in the readme, or a wiki somewhere, or an automated PR-closing bot; but it would be a lot easier all round if GitHub let you replace the PR tab with some text so there was a consistent place to look for this information.