Start with a random metal track in mind an I find I get a band mix suggestion, a nordic folk metal mix, 11 more tracks from the same band, 3 tracks I listened to very recently, 4 connected other artists and "5 CREEPIEST Old Found Footages - Recommended for you" as a perfect example of the crap being injected recently. They must notice I've never once clicked one.
I do clear cookies and history periodically, so perhaps not having a multiple year history counts against.
In that other link I posted YT suggests pruning your watched history to get better recommendations -- but that's a huge undertaking given the thousands of garbage videos that I've clicked out of in five seconds. Seems like it would be better if the algorithm only operated on data from videos a user manually "likes".
I tried it out just now for an album I like. I get about 25% unrelated junk or stuff I've already watched, but the rest are also full albums that seem like fairly solid recommendations based on the album at hand (Jan Jelinek's "Loop Finding Jazz Records").
https://www.youtube.com/watch?v=6hIgBEXuQD8