I have been marking "not interested" in every "vtuber" related video it has recommended to me, and I don't remember clicking on a single one, but it still hasn't stopped recommending them to me.

Your graph of interests merges with "people who watch vtubers" too strongly, so the recommendation engine keeps pushing that stuff to you.

I'm assuming that a downvote with a negative sentiment comment is the most severe signal you can send.