No, they do. Videos that you've watched are marked as such. They have a red bar at the bottom of the video thumbnail that shows how much of the video you've watched. For ones you've watched all the way through, it's just a solid red bar.
After a few months, YouTube just forgets. I can go to, say, Chubbyemu's channel (https://www.youtube.com/c/ChubbyemuGames/videos), which I've watched most videos on, and it only shows that I've watched videos 6 months or newer. YouTube forgets everything past that.
The point of my post was multi-fold. YouTube will recommend a video they _know_ you've watched; it's got the red bar at the bottom. But even if they didn't do that, they'll forget in six months anyway and recommend it anyway.
> Some people, like me, will watch the same video multiple times, if not dozens.
Hmmm, yeah, you're right. I do the same. Must be that their AI has learned that behavior, but hasn't quite figured out what makes a video "rewatchable" yet.