On the heels of the release of the January 22 public release of iOS 12.1.3, Apple has stopped signing previous versions, iOS 12.1.1 and iOS 12.1.2. This is a routine happening when Apple releases operating system upgrades.
This means owners of iOS devices that have upgraded to iOS 12.1.3 will no longer be able to downgrade to the previous versions of iOS 12.
Apple is known to stop signing older versions of operating systems after it releases a public update, as part of an effort to encourage users to keep their devices’ operating systems up to date.
iOS 12.1.3 is currently the most recent publicly-available version of iOS available for iPad and iPhones. A beta of iOS 12.2 is currently available to developers and public beta testers. A public release of iOS 12.1.4 is expected this week, to fix a major FaceTime privacy issue.