Apple stopped signing legacy iOS 11.3 code
A week after iOS 11.3.1 was released, Apple officially closed iOS 11.3 system authentication channel. Apple usually closes the authentication channel of the old system shortly after the new system is launched. According to Apple, users are encouraged to use the latest version of the system. This also means that once users upgrade to iOS 11.3.1 they will no longer be able to fall back to iOS 11.3.
Apple is currently testing the iOS 11.4 system. Not surprisingly, it should be officially released before the WWDC 2018 conference in June.
If you have upgraded to iOS 11, it is recommended that you update it to the last version.