Apple
Apple Stops Signing iOS 15.4.1 Following iOS 15.5 Release
May 16 release of Apple Stops Signing iOS 15.4.1, the previous version of iOS that was released in late March. Because iOS 15.4.1 is no longer signed, there is no way to revert to that version of iOS after installing iOS 15.5.
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Apple Stops Signing iOS 15.4.1 Following iOS 15.5 Release

Apple Stops Signing iOS 15.4.1 Following iOS 15.5 Release
Apple typically ceases signing older versions of software updates after new releases are released in order to encourage customers to keep their operating systems up to date, so iOS 15.4.1’s absence is not surprising.
Following the release of iOS 15.4, a small update was released to remedy an issue that could cause the iPhone’s battery to deplete faster. It also fixed an AppleAVD flaw that might allow an app to run arbitrary code with kernel privileges, which Apple claimed was being actively abused.
Apple Cash, a feature for requesting and transferring money, was added to iOS 15.5 along with additional features for Podcasts and a fix for a problem with home automation. It also contained a number of security fixes, so anyone who hasn’t yet upgraded to iOS 15.5 should do so.