Following iOS 15.4.1 release Apple now stopped signing iOS 15.4 preventing downgrade on devices.
Apple now no longer signs iOS 15.1.1 preventing downgrade from iOS 15.2.
Apple now prevents downgrade from iOS 15 and iPadOS 15. This means that during restore operation, device will receive iOS 15 or iPadOS 15 accordingly. This seemingly would improve iOS adoption rate.
Having no other iOS 14.x update, any previous version will now be only updated to iOS 15 too.
This needs to be taken into consideration for test devices.