Apple recently released their iOS 15.4 updates and now they have stopped signing iOS 15.3.1. What this means is that if you have updated your device to iOS 15.4, you will no longer be able to downgrade it to the previous version.
Apple normally does this with a major new release, when the previous version had some issues, normally security vulnerabilities, etc.
The iOS 15.3.1 update fixed a security vulnerability in Apple’s Webkit, the iOS 15.4 update brought a range of new features to the iPhone.
The iOS 15.4 software update included a new feature for Apple’s Face ID software, you can now use Face ID to unlock your iPhone when you are wearing a face mask. This feature will only work on the iPhone 12 line up and the iPhone 13 line up.
The update also introduced a new range of Emoji to the iPhone and a range of other new features for the iPhone. Apple also released iPadOS 15.4 for the iPad.
The iPadOS 15.4 update brings a range of new features to the iPad, this includes the Universal Control feature that was supposed to land with iOS 15.
Universal Control allows you to use the mouse and keyboard from your Mac with your iPad when they are next to each other, you can easily switch between the two devices.
The iOS 15.4 software update is now available to download and you can install it by going to Settings > General > Software update. If you download this update you will no longer be able to downgrade to iOS 15.3.1.