A number of YouTubers have made the claim that their views were affected but not revenue, so it seems like the monetization is based on ad-watching views at least.
If the monetization weren't limited to ad-watching views, we'd probably still be trying to figure out what happened.
In short, age estimation will restrict videos from viewers, and a creator has almost no way of knowing if a video was age-restricted or not.
Bellular has a video about the situation: https://www.youtube.com/watch?v=jSYLe6Yq4R4
I wonder if CTR was affected. Could one of the affected channels could have detected that not adding up? I guess it was probably already blocked for privacy. Maybe I shouldn't be giving them ideas.
Interestingly, anybody can now measure what percentage of any channel's viewers run ad blockers, by using publicly available data on how much their views dropped during this period.
Turning a profit on video outside YouTube is a far more difficult undertaking.
My point: This problem is far worse when a monopoly is involved.
Especially in the scenario that (as the top level comment in this thread suggests) YouTube didn't actually make any changes and the reason the views dropped is because EasyList added an entry to their privacy filter. Should YouTube have recognized that they're in a quasi-monopoly position as you suggest, done the research to identify EasyList as the culprit behind the view metric drop, and then released a change to their client to add a new endpoint which isn't blocked by EasyList?
We don't know that the EasyList theory is what's really going on here, but if you're going to tar YouTube/Google over this ordeal, then I think you have some responsibility for suggesting how they could have done better.