Apple has stopped signing iOS 13.3.1 after the release of iOS 13.4, stopping people who have upgraded to the latest version from going back again.
Every iOS update Apple introduces is designed to be speedier, more stable, and more secure than its predecessor. As a result, Apple blocks downgrades soon after its latest release has rolled out to everyone. This was the turn of iOS 13.3.1 to go to the big software graveyard in the sky!