Today Apple stopped signing iOS 15.3.1 after releasing a major version update for iOS 15.4 on March 14. Since iOS 15.3.1 is no longer signed, if you’ve already updated to iOS 15.4, it’s not possible to downgrade to that version of iOS.
Apple usually stops signing software updates for older versions after a new version is released to encourage customers to keep the operating system updated, so it’s not uncommon for iOS 15.3.1 to no longer be signed.
iOS 15.3.1 was a bug-fix update addressing a WebKit vulnerability, while iOS 15.4 was a major update that added new emojis, Face ID support for masks, and a ton of other features. Apple has also stopped signing tvOS 15.3.