Interestingly, iOS 15 is the first version of iOS in history to be optional if you want security updates. You will be able to choose if you want to go to iOS 15 and get the new features (including the CSAM prevention stuff), or you can stay on a security update only channel for iOS 14 (for a unknown period, but I'd guess until WWDC 2022? N-1 seems reasonable).

https://www.apple.com/ios/ios-15-preview/features/ (under settings)