Unfortunately, it'll pick up recommendations from watching one thing one time, so watching that one video your weird uncle sent you is enough to pollute your algorithm with his weirdo shit.
* Copy URL
* Open new private window
* Paste URL: trim pp and/or si parameters, which are a form of tracking
New prank on someone that left their laptop unlocked. Open YouTube to something they hate and then close it before they get back.
But at least you still have that ability compared to most platforms.
I haven't paid close attention to the effects, because I don't thumbs down too many videos. But it did feel like I generally don't get that channel as much after thumbing down a video from my recommended feed.