iPhone, iPad, and iPod touch owners who have upgraded to iOS 12.1.3 will no longer be able to downgrade to iOS 12.1.1 or iOS 12.1.2.
iOS 12.1.3, available for all iOS devices able to run iOS 12, was a minor bug fix update that addressed issues with the HomePod, iPad Pro, CarPlay, Messages, and more.
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 12.1.3 is now the only version of iOS that can be installed on iPhones and iPads, but developers and public beta testers can download iOS 12.2, a major update that's currently being beta tested.
Apple will also soon release iOS 12.1.4 to fix a major FaceTime privacy bug.