Apple is delaying this, I suspect, because it confuses the privacy message: If every time they talk about privacy someone can make them look like hypocrites by pointing out the on-device scanning, well that's just ugly for Apple. I suspect the on-device scanning element is going to be removed and it'll be on ingress to iCloud, which is what I said on my very first post to this. It was ill-considered to do it on device given Apple's privacy push.