I basically never rewatch videos I watched, unless I search for a specific one, yet Youtube consistently shows me tons of videos I watched, with full red bar and everything. First I thought just not paying attention to them would slowly teach the algorithm that I'm just not interested, then I thought marking each such video as "not interested" would help. I gave up and installed "YouTube: Hide Watched Videos - by Ev Haus" script into Tampermonkey to finally get rid of those, and now about 50% of my Youtube home screen is empty space.