Following the launch of iOS 16.3.1 on February 13, Apple is no longer signing iOS 16.3, the previously available version of iOS. Now that Apple has stopped signing iOS 16.3, it is no longer possible to downgrade to that version of iOS after installing the iOS 16.3.1 update.

Apple routinely stops signing older versions of iOS after new releases come out in order to encourage customers to keep their operating systems up to date, so it is not unusual that the iOS 16.3 update is no longer being signed.

iOS 16.3 was a relatively minor update that introduced support for physical security keys for Apple IDs, expanded Advanced Data Protection globally, added support for the second-generation HomePod, and more.

While iOS 16.3.1 remains the current public version of iOS, Apple released the first beta of iOS 16.4 last week with support for new emoji characters, web push notifications, and more.

Source: Macrumors