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. Apple continues to sign iOS 17.0.2 for now.
Notably, this means that it is no longer possible to downgrade an iPhone from iOS 17 to iOS 16. It is not possible to downgrade to iOS 16.7.
Apple routinely stops signing older iOS releases over time in order to prevent users from downgrading to previous software versions.
Source: Macrumors