> the new owner can't spy on my viewing

Quick question: how do you distinguish content blocker extensions from regular extensions? All the Adblockers I know have to use both to be effective at hiding ads, especially on YouTube.

In that case, the new owner can still do the spying.

In iOS at least, it tells you in Settings > Safari > Extensions. E.g. if I tap into one of the AdGuard ones I see:

“"AdGuard — General" does not have permission to read or transmit content from any web pages.”

Ones that have more permissions will say so.