Apple Stops Signing iOS 15.6 Following iOS 15.6.1 Release, Downgrading No Longer Possible
Following the launch of iOS 15.6.1 last Wednesday, Apple has stopped signing iOS 15.6, the previously available version that came out in mid-July. Since iOS 15.6 is no longer being signed, it is not possible to downgrade to that version of iOS after installing iOS 15.6.1.

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 15.6 is no longer being signed.
iOS 15.6 was minor in scale, introducing an option to restart, pause, rewind, and fast-forward a live sports game that's in progress, plus it addressed an issue that could cause the Settings app to continue to display that device storage is full even when it's not.
Released on August 17, iOS 15.6.1 had no new feature additions, but it does include a critical security fix that addresses kernel and Webkit vulnerability that could lead to arbitrary code execution.
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