Apple has stopped installing iOS 15.1. Users will not be able to Downgrade, Signing, or Restore back to that iOS version.
Disable downgrade back to iOS 15.1
Apple has disabled the installation of the following iOS, iPadOS versions.
That means the user Unable to Downgrade, Signing, Restore or Install Firmware iOS 15.1 (iPadOS 15.1) At this time, it can only install iOS 15.1.1 (1 Dec. 64)
As for iOS 15.1 users, it can still be used normally. But if updated to iOS 15.1.1, it will not be able to downgrade back to iOS 15.1.
iOS 15.1.1 was previously released only to iPhone 12, iPhone 13 users. Fix the problem of dropped calls during a call