According to the latest report, after the official version of iOS 15.4.1 was released on March 31, Apple has stopped signing the official version of iOS 15.4, the previous version of iOS launched in early March.
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Since iOS 15.4 is no longer signed, once you have installed iOS 15.4.1, it is not possible to downgrade back to the previous iOS version. Apple typically stops signing software updates for older versions after new versions are released, to encourage customers to keep the operating system updated.
Moreover, the official iOS 15.4 release is a major update that features support for unlocking the iPhone while wearing a mask, new emojis, iPad support for universal controls, and more.
Furthermore, iOS 15.4.1 added fixes for iPhone battery drain issues, but more importantly, it addresses an AppleAVD issue that could allow an app to execute arbitrary code with kernel privileges.
Apple says the vulnerability may have been actively exploited, so updating to iOS 15.4.1 is the best option. In addition, Apple also stopped signing tvOS 15.4 and HomePod 15.4 software.
