After iOS 15.4.1 is released, Apple stops signing iOS 15.4, downgrading is no longer possible

Following the March 31 release of iOS 15.4.1, Apple has stopped signing iOS 15.4, the previously available iOS version released in early March. Because iOS 15.4 is no longer signed, once you have installed iOS 15.4.1, you cannot downgrade to that version of iOS.

Apple typically stops signing older versions of software updates after new versions are released to encourage customers to keep their operating systems up to date, so it's not unusual to no longer sign iOS 15.4.

iOS 15.4 is a major update with support for unlocking iPhone while wearing a mask, new emoji characters, universal controls on iPad, and more.

iOS 15.4.1 adds a fix for battery drain issues on iPhone, but more importantly, it resolves an AppleAVD issue that could allow an app to execute arbitrary code with kernel privileges. Apple says this vulnerability may be actively exploited, so it's a good idea to update to iOS 15.4.1 if you haven't already.

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