People on Twitter have mentioned that this probably extends private set intersection where each party learns the intersection of a set without learning either i

People on Twitter have mentioned that this probably extends private set intersection where each party learns the intersection of a set without learning either input set, but that wouldn't quite be enough. If you assume there is an algorithm where Google can provide some "encrypted" form of their "users who have seen this ad" list such that the amount spent can be computed without knowing the intersection, you could exclude people who have opted out from the list Google inputs into the algorithm.