https://wccftech.com/apple-no-longer-signing-ios-15-3-downgrade-from-ios-15-3-1-stopped/

Apple has just stopped signing iOS 15.3 and iPadOS 15.3 firmware for iPhone and iPad, stopping downgrades from 15.3.1.

Just a few days ago, Apple released iOS 15.3.1 and iPadOS 15.3.1 to the public with several bug fixes. The update was released just a couple of weeks after iOS 15.3 and iPadOS 15.3 was seeded to everyone around the globe.