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 No Longer Downgrade iOS 15.4 or iPadOS 15.4 to iOS 15.3.1 or iPadOS 15.3.1 on iPhone and iPad
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.
If you have already updated to version 15.4 then you have to stick with it until a newer version comes along. But honestly, there's no reason why you would want to stay on older firmware given how many new features you gain compared to iOS 15.3.1 and iPadOS 15.3.1, such as Universal Control.
At this point, there's no reason to attempt a downgrade at all. If you download the firmware file and try restoring your device using iTunes or Finder, it will simply fail. It's simply a good idea to upgrade to iOS 15.4 or iPadOS 15.4 at this point. If interested in that, go to Settings > General > Software Update. Let the page refresh and then tap on Download and Install.
While you are here, check out the following: