Following the May 16 launch of iOS 15.5, Apple has stopped signing iOS 15.4.1,
the previously available version of iOS that came out in late March. Because iOS 15.4.1 is no longer being signed, it is not possible to downgrade to that version of iOS after installing iOS 15.5.
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, so it is not unusual that iOS 15.4.1 is no longer being signed.
iOS 15.4.1 was a minor update that addressed an issue that could cause the iPhone’s battery to drain more quickly following the iOS 15.4 launch. It also addressed an AppleAVD issue that could allow an application to execute arbitrary code with kernel privileges, which Apple said was actively exploited.
iOS 15.5 added an Apple Cash feature for requesting and sending money, along with new features for Podcasts and a fix for an issue with home automations. It also included several security updates, and those who have not already upgraded to iOS 15.5 should do so for that reason.