Let's guess, they rewrote the clock app using SwiftUI (probably without the original main developer around), because like, how hard/risky could it be?

https://www.macrumors.com/2023/10/04/apple-stops-signing-ios...

Following the release of iOS 17.0.3 earlier today, Apple has stopped signing iOS 16.6.1, iOS 17, and iOS 17.0.1, preventing iPhone users from downgrading to any of those software versions.