Apple has stopped signing iOS 13.3.1 following the release of iOS 13.4 last week. This means users running iOS 13.4 can no longer downgrade back to earlier version.
iOS 13.4 brings a long-tail list of new and improved features, including iPad trackpad support, iCloud Drive folder sharing, and more. With Apple no longer signing iOS 13.3.1, users cannot install iOS 13.3.1 on their devices, which ceases hopes for a jailbreak. If you’re already on iOS 13.4, then you’re locked to that firmware now unless a new software update is available.
Apple doesn’t allow you to restore earlier versions of iOS due to security reasons. It recommends using the latest software version for more secure experience.