Apple has stopped signing iOS 13.3.1, preventing users from being able to downgrade or restore to that firmware.
Currently, Apple’s latest firmware is iOS 13.4. If you accidentally update or restore, you’ll find yourself on that version. Developers have been seeded with iOS 13.4.5 beta.
Those interested in jailbreaking are typically advised to stay on the lowest firmware possible; however, the checkra1n jailbreak makes it possible to jailbreak most devices between the iPhone 5s and iPhone X on any firmware version above iOS 12.3. The unc0ver jailbreak supports newer devices running iOS 13.3 or lower.