I think this is actually motivated by upcoming EU legislation that would mandate CSAM filtering by cloud storage providers. They tried hard to engineer a soluti

I think this is actually motivated by upcoming EU legislation that would mandate CSAM filtering by cloud storage providers. They tried hard to engineer a solution that would provide some guarantees, but doing the processing client-side sets an even more damaging precedent. The same guarantees might have been doable purely server-side using homomorphic encryption but that tech is still very nascent and not deployed at the scale of iCloud.