Apple officially announced today that it has stopped signing iOS 16.3 updates. This means that users who have already upgraded to the iOS 16.3.1 update (released on February 13) will not be able to downgrade to that version.
Apple will stop signing older versions of iOS after a period of time to encourage users to upgrade their systems and make sure they are up to date.
For those interested, you can visit the links below to read about the key features of iOS 16.3 and iOS 16.3.1 updates.