After the official release of iOS 13.4, Apple today stopped the verification of iOS 13.3.1, which means that users can no longer downgrade to iOS 13.3.1.
Apple added memoji stickers in iOS 13.4, shared iCloud disk folders from the Files app, universal purchases, and CarPlay dashboard support for third-party navigation.
Previous information states that iOS 13.3.1 added a setting to control the use of location services by the U1 ultra-wideband chip, which solved the problem of a short delay before editing Deep Fusion photos taken on iPhone 11 or iPhone 11 Pro.
For some users who have updated to the latest iOS system, when they encounter major bugs, the downgrade method is sometimes very effective. This is why Apple will wait for a while before closing the old iOS system verification channel.