Apple has discontinued signing iOS 15.5, the previous version of iOS that was accessible and was released in the middle of May, as of the launch of iOS 15.6 on the prior Wednesday. After upgrading to iOS 15.6, it is not feasible to perform a backwards compatibility update to iOS 15.5, as that version of iOS is no longer being signed.
It is generally unusual for Apple to discontinue signing older versions of software updates once new releases have been made in order to encourage customers to keep their operating systems up-to-date. Because of this, the fact that iOS 15.5 is no longer being signed is not surprising.
iOS 15.5 was a very minor update that included enhancements to Apple Cash in the Wallet app as well as a new setting that provides users of podcasts with the ability to limit the number of episodes that can be saved on their respective devices.
Released on July 20th, iOS 15.6 also brought some minor changes and added a few new capabilities, like the ability to pause, rewind, and fast-forward during an ongoing live sports game on the device. Some minor problems were also fixed, including one that might have led the Settings app to falsely indicate that the device’s storage was full when it wasn’t.