Following the release of iOS 13.1.2 on Monday, Apple has stopped signing several previous versions of iOS, preventing downgrades to these earlier versions. iOS versions no longer being signed include iOS 12.4.1, iOS 13.0, and iOS 13.1.1, while iOS 13.1 remains signed for the time being.
iOS 13.1.2 was a bug fix release that quickly followed iOS 13.1.1, which primarily addressed issues with battery drain, Siri, and access permissions for third-party keyboards. iOS 12.4.1 was largely a jailbreak fix released by Apple in late August.
Apple routinely stops signing older versions of software updates after new releases come out in order to encourage customers to keep their operating systems up to date.
iOS 13.1.2 is now the current public version of iOS, but developers and public beta testers can download iOS 13.2, an upcoming update with features like the Deep Fusion image processing system for the new iPhone 11 lineup.