Following the launch of iOS 16.3 on January 23, Apple is no longer signing iOS 16.2, the previously available version of iOS. Now that Apple has stopped signing iOS 16.2, it is no longer possible to downgrade to that version of iOS after installing the iOS 16.3 update.
Apple routinely stops signing older versions of iOS after new releases come out in order to encourage customers to keep their operating systems up to date, so it is not unusual that the iOS 16.2 update is no longer being signed.
iOS 16.2 was a significant update that added Apple Music Sing, the Freeform app, Advanced Data Protection for iCloud, and more. iOS 16.3 introduces Security Keys for Apple ID, changes to Emergency SOS, and it adds support for the new HomePod model.
Apple is expected to release the first beta of iOS 16.4 in the near future, but as of now, iOS 16.3 is the only available version of iOS 16. Apple has also stopped signing iOS 15.7.2 and iOS 12.5.6 following the release of iOS 15.7.3 and iOS 12.5.7 security updates earlier in January.
Source: Macrumors