> Do you mean this would apply to Macs too?

From [0]

'These features are coming later this year in updates to iOS 15, iPadOS 15, watchOS 8, and macOS Monterey.'

So yes.

Also by the OP:

> iMessage encrypts messages end-to-end by default; however, if you have iCloud backup turned on, your messages can be accessed by Apple (who has the keys for iCloud backups) and, by extension, law enforcement with a warrant.

So if you use iMessage and iCloud Photos then they will be scanned by Apple. Won't be surprised to see the on-device scanning feature more prevalent on M1/M1X Macs.

[0] https://www.apple.com/child-safety/