Apple stopped signing iOS 15.6, the previous version that was released in mid-July, with the release of iOS 15.6.1 last Wednesday. It is not feasible to downgrade to iOS 15.6 from Apple after installing iOS 15.6.1 since that version of iOS is no longer being signed.
In order to encourage users to keep their operating systems up to date, Apple typically ceases signing older versions of software updates after new releases are made, so the fact that iOS 15.6 is no longer being signed is not unusual.
A small update, iOS 15.6 added the ability to resume, pause, fast-forward, and rewind an ongoing live sports event. It also fixed a bug that might allow the Settings app to falsely indicate that the device’s storage is full even when it isn’t.
Despite not having any new features when it was released on August 17, iOS 15.6.1 does contain a crucial security update that fixes a kernel and Webkit vulnerability that could result in arbitrary code execution.