Apple has now stopped signing iOS 13.3.1 firmware recently after the beta release of iOS 13.4.5 with many more new features.
That is you are no longer able to downgrade or restore your iPhone or iPad to that firmware version (iOS 13.3.1) from a new beta of iOS 13.4.5 and iOS 13.4.
So you have no chance to downgrade back to lower version anymore. If you want to restore your device from iTunes or OTA, then you will force to install Apple’s current latest firmware iOS 13.4.
And those who have accidentally updated your device to iOS 13.4 or later via OTA or iTunes, downgrading back to iOS previous versions like iOS 13.3.1 or lower is not possible.
Apple always gives older versions of iOS to restore whenever it releases a new version so that people can have the ability to downgrade their devices to the previous version if they are not satisfied with the new one or any issues. But after some time, it stops signing the previous version and allows the latest iOS version only.
You can also visit this link https://ipsw.me to check the signing status of Apple firmware.
Also, there are lots of users that are going to be affected by this change, specially Jailbreakers. The Jailbreak hackers usually work on to build a jailbreak on older versions.
So Jailbreak lovers who are on iOS 13.3 or below, do not update to the iOS 13.4 or later but if you do not care about Jailbreak, you can upgrade to iOS 13.4 or latest beta 13.4.5 anytime.