I'm comfortable with Tampermonkey scripts and writing my own JavaScript, I just can't reasonably comprehend YouTube's generated HTML structure and constant changes enough to know how to write this myself.
What I want is essentially something I can turn on or off that will completely hide videos I've already seen from the suggestions page, the recommended sidebar next to videos and just generally the homepage/the site. It's not that I don't like seeing videos again, it's just the complete burial of suggestions from YouTube of videos I haven't seen among ones I have.
Best Answer
Here's a decent script that'll do just what you asked: https://greasyfork.org/en/scripts/13040-youtube-hide-watched-videos