Following the release of iOS 15.5 on May 16, Apple has stopped signing iOS 15.4.1, the previously available iOS version released in late March. Because iOS 15.4.1 is no longer signed, you cannot downgrade to iOS 15.5 after installing that version of iOS.
Apple typically stops signing older versions of software updates after a new version is released to encourage customers to keep their operating systems up to date, so it's not unusual for iOS 15.4.1 to no longer be signed .
iOS 15.4.1 is a small update that resolves an issue that could cause iPhone batteries to drain faster after the release of iOS 15.4. It also resolves an AppleAVD issue that could allow an application to execute arbitrary code with kernel privileges, which Apple says is being actively exploited.
iOS 15.5 adds Apple Cash functionality for requesting and sending money, as well as new features for Podcasts and fixes for home automation issues. It also includes several security updates, so users who haven't upgraded to iOS 15.5 yet should do so.
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