alt Hacker NewsNote, though, that disabling PRs in particular is only a couple months old (and was prompted by the current wave of consternation around AI-assisted contributions). Before that you had to set up a bot that would just auto-close them and it still wasn't obvious in the UI that this was how it worked.
This also is mostly a bandaid, but my recollection is that for years there's been support for picking a different email for notifications for specific repos (and if not, I'm positive there has been a setting per org at least for that). Using a throwaway email for notifications for a repository that you don't care about also seems like a pretty reasonable way to use it as a place to dump code without needing to care about being externally reachable.
Maybe people will see the giant list of issues and PRs and be concerned, but if you put a note at the top of the README indicating that's what's going on, the only people who won't notice (or won't accept it) it are the ones that you're probably trying to avoid with this strategy anyhow, so it's no loss to have them go elsewhere.