Like an ad blocker, but something that prevents modals/pop-ups from popping up on screen and annoying everyone. It could also block ads as those are often modals telling you to turn off ad blocker.
Many ad-blockers are configurable so you can use blocking on some things only. With “uBlock Origin”, you can right-click and say “Block Element” and then select exactly what annoying piece (or hierarchy of pieces) should be removed on that site. This works pretty well when sites use something generic for their blocking backgrounds like a "#modal_overlay" ID or something. Some sites are more tricky and use IDs and CSS classes that appear to be randomly-generated.
One of my pet hates and they seem to be getting more prevalent all the time. I say, vote with your feet (or 'finger') and just click away from sites as soon as a modal overlay pops up. Maybe, if we all did that, the site owners would stop this unbelievably annoying practice.