No idea how relevant this is, but with Advanced Data Protection, there's likely to be a bit more awareness from users about E2EE.
CloudKit APIs appear to already support this from an app layer perspective, and ADP should add encrypted client device backups to complement this. Not really seen enough to tell whether you'll get encrypted "app" data stores on iCloud drive, or if you still need to use CloudKit though - imagine that will become clear once more people are using ADP.