I suspect they're going to soon do what Amazon did as well, where they start putting ads into the regular YouTube Premium service, and charge an extra $3 a month for a completely ad free experience.
I have the family plan shared across six accounts, and it went to $26, which really isn't that much but I'm not entirely sure why they're doing it.
This has happened to me btw. I can only suspect they are "testing" the reception.
Why would they? The entire point of YouTube Premium is removing ads. There are a few other benefits, but being ad-free is the big one. If they put back ads, I believe that most people will simply cancel their subscription and get a renewed interest in ad-blockers. It makes more sense to just increase the price, as they do.
It is not like Amazon. Most people get Amazon Prime for the "free" shipping, and Prime with ads is a good value proposition, you get shipping but get a discount on the part that doesn't interest you. I don't get why tying a shipping to a streaming service isn't more controversial by the way, it is borderline illegal.
Oh, and by the way, ad-free is not really ad-free, you still have sponsored segments, but these are not under YouTube control.