Following the release of iOS 12.4 on July 22, Apple has stopped signing iOS 12.3.1 and iOS 12.3 the previous versions of iOS that were available to consumers. Apple is also no longer signing iOS 12.3.2, an update exclusive to the iPhone 8 Plus.
iPhone, iPad, and iPod touch owners who have upgraded to iOS 12.4 will no longer be able to downgrade to iOS 12.3, 12.3.1, or 12.3.2.
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 12.4 is now the only version of iOS that can be installed on iPhones and iPads, but developers and public beta testers can download iOS 13, an upcoming update that's currently being beta tested.