Well, i can understand the comments, because those things can be used, in theory, to bait and manipulate kids. I can even sorta see the logic (as exaggerated and extreme as it is) behind turning off the subscription bell, but other than that, most of the removals make no sense.
As for the adult ads, they work by tracking what you watch and look at, and giving you ads for things you are more likely to buy. For example, if you spend alot of time watching lego videos, and visiting lego-centric sites, and searching lego related topics on google, they will show you an ad for lego, rather than say.... the ninja blender, because you are more likely to buy that product, and the advertiser (lego) makes more money, which trickles down to the advertisee (google/YT). They alter the ads to "target" you, hence the term "targeted" ads. It's almost uncanny how accurate they can be about location, age, intrests, it's both incredible and terrifying at the same time. That's why the ad revenue from videos marked "for kids" will drop, google/yt is making less money, therefore, they can't afford to pay the creators as much.