Following the release of iOS 13.1.2 last week, Apple today stopped signing iOS 13.1, meaning downgrading to this version of iOS is no longer possible. 

Apple last Friday stopped signing iOS 12.4.1, iOS 13.0, and iOS 13.1.1, but continued signing iOS 13.1 until today. 

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. 

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.

Source: MacRumors