Search engines will do literally anything except the option "never show results from this domain again"

Is there something obvious I'm missing that makes it infeasible, or maybe is it just something only I want?

As for this site there's too many tags for them to be useful imo. Give it 2 weeks of using the search engine and I bet you could hide silly fake tags in there and I'd never notice. Lots of tags = no tags.

I was picturing maybe a little pillbox type thing you might find appended to Google search results.

For instance when a result is a PDF: https://img.imgy.org/-7lq.jpg