How do I filter search results for PRs with deleted head respositories?

I am trying to filter search results for PRs with deleted head repositories such that it says “unknown repository” in the PR:

Screenshot from 2020-10-12 09-06-06

There doesn’t seem to be any information on this in the documentation:

Hey @aaronfranke so for the filter of search results, what you’re asking for isn’t quite possible. It would be amazing if you could submit your idea through our feedback form:

However, I’m wondering if you can’t accomplish what you’re trying to do by adjusting your search itself, rather than filtering the results you get?

Using something even as basic as is:pr "unknown repository" (along with other qualifiers) might be a decent start, maybe?

Let me know what you think, and if that doesn’t get you on the right path. Would be curious to know if I’m misunderstanding your intent.