https://wccftech.com/ios-15-3-1-no-longer-signed-by-apple-downgrade-from-ios-15-4-closed/

Thinking about downgrading from iOS 15.4 or iPadOS 15.4 to iOS 15.3.1 or iPadOS 15.3.1, respectively? Not so fast.

You can simply no longer downgrade iOS 15.4 or iPadOS 15.4 to an earlier version of Apple's iPhone or iPad software. The reason for this is rather simple - Apple is no longer signing any other firmware apart from iOS 15.4 and iPadOS 15.4.