To play advocate of the devil here (I can feel the down votes and Foss brigade already coming) : I can understand their logic. Their whole business is fucking ads. These apps bypass their entire business. Count yourself lucky they've ignored them this long.
YouTube premium is actually one of the only subscription services I use. My entire family gets ad free YouTube + YouTube premium. The amount of YouTube we watch and music we listen... 100% worth it. + a premium view supports creators more than a free view, so I even get to feel a bit better about myself. (I watch too many different channels to make it financially viable for myself to send them all a dollar/month through means outside of YouTube)
I'm not arguing Google is a holy company and inheritely good. It's just that they offer a service and in the capitalist market we live in, they want/need to be compensated. If you don't like it, vote with your wallet. I can live with the price I pay to use YouTube. It gives me just enough joy in life that I find the monetary fee reasonable. If they would jack up the prices, I might say fuck it and find alternatives.
My whole point is that this blocking of ad blocking apps is not a surprise at all, and even understandable from a capitalist company pov.
That doesn't mean I support capitalism and assholes making life worse for us plebs. It just means that in this particular situation, it fits with my lifestyle as a cog in the machine. It's too much of a pita for me personally to "pirate" YouTube vids like I do with other media content.