This is the same feature that 
added here several months ago, where file hashes are scraped and compared to a database of known child exploitation images.
The thing that has people concerned with Apple is that now it won't just be images in your iCloud, but also those stored locally on users' own devices.
#cososec
Apple will scan photos stored on iPhones and iCloud for child abuse imagery - The Verge
https://www.theverge.com/2021/8/5/22611305/apple-scan-photos-iphones-icloud-child-abuse-imagery-neuralmatch
@voltronic Well, GDI, Apple.
I understand the idea; but Apple was one of the few companies that actually took user privacy seriously!
While I want all child predators caught - this is a worrying precedent from the company that refused to decrypt phones from even known terrorists!