Apple has ceased signing iOS 15.3, the previously accessible version of iOS that was released in late January, with the release of iOS 15.3.1 on February 10. Because iOS 15.3 is no longer signed, it is not feasible to revert to that version of iOS after updating to iOS 15.3.1.
Apple often de-signs older versions of software updates upon the introduction of new ones to encourage users to keep their operating systems current.
iOS 15.3 was a big patch update that fixed a serious Safari problem that allowed browsing activity to be leaked, while iOS 15.3.1 was another bugfix release that addressed further WebKit vulnerabilities.
Due to the suspension in code signing, iOS 15.3.1 is the only software update accessible to users of iPhones and iPads. Apple has begun beta testing iOS 15.4, which features Face ID enhancements and lays the groundwork for Apple’s future Tap to Pay With iPhone functionality.