Following the release of iOS 15.3.1 on February 10, Apple has stopped signing iOS 15.3, the previously available version of iOS that came out in late January. Because iOS 15.3 is no longer being signed, it is not possible to downgrade to that version of iOS if you've updated to iOS 15.3.1.

Apple routinely stops signing older versions of software updates after new releases come out in order to encourage customers to keep their operating systems up to date.

iOS 15.3 was a bug fix update that addressed a significant Safari bug that could leak browsing activity, while iOS 15.3.1 was another bug fix update further addressing WebKit vulnerabilities.

Source: Macrumors