Apple has stopped signing iOS 15.3.1, the previously available version of iOS that was released in February, following the release of iOS 15.4 on March 14. If you’ve updated to iOS 15.4, you won’t be able to go back to iOS 15.3.1 because it’s no longer being signed. Apple published Apple is no longer signing iOS 15.3.1 & iPadOS 15.3 last week, which includes bug fixes and security enhancements for iPhones and iPads. As a result, we expected Apple would end the signing window for the older iOS & iPadOS 15.3.1 firmware soon.
Following the release of iOS 15.4, Apple has stopped signing iOS 15.3.1, making it impossible to downgrade.
Apple frequently ceases signing previous versions of software updates after new releases are released in order to encourage customers to keep their operating systems up to date, so iOS 15.3.1 no longer being signed is not surprising.
iOS 15.3.1 was a bug patch release that addressed WebKit vulnerabilities, but iOS 15.4 was a big upgrade that included new emoji characters, Face ID with mask support, and a slew of other improvements. tvOS 15.3 has likewise been designed by Apple.