As is traditional, Apple has ceased the signing of iOS 12.0.1, following the October 30 release of iOS 12.1. This means that downgrading from iOS 12.1, is no longer possible.
iPhone, iPad, and iPod touch owners who have upgraded to iOS 12.1 will no longer be able to downgrade to iOS 12.0.1. The iOS 12.1 update was significant, introducing Group FaceTime, eSIM support for Apple’s newest iPhones, new emoji, and more.
Apple routinely stops signing older versions of iOS following software updates. They do this to encourage their customers to update to the latest versions of the operating system.
iOS 12.1 is now the only version of iOS available to the general public for use on their iPhone, iPad, and iPod touch devices. iOS 12.1.1 is currently being tested by developers and public beta testers.