Couldn't that affect third party sponsorships, though? Both getting them and reporting numbers to existing ones?

Presumably, it would affect that, and also long-term channel growth. Which would be dastardly if it were intentional, because it would basically cull the platform of channels who voice support for ad blocking.

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.