Apple today has officially stopped signing iOS 12.0.1, which was originally released back at the beginning of October. This means that downgrading from the most recent version of iOS, which is iOS 12.1, is no longer possible.
Apple regularly stops signing older versions of iOS as part of its efforts to keep users on the most up-to-date and therefore secure versions of the operating system. Apple most recently stopped signing iOS 11.4.1, preventing downgrades from iOS 12 to iOS 11, and stopped signing iOS 12.0.
Over the years, the reasons to downgrade have become increasingly sparse – especially as users rely on over-the-air updates versus plugging directly into iTunes. In the past, users would often downgrade in order to jailbreak.
The most current public version of iOS is iOS 12.1. Apple is also currently beta testing iOS 12.1.1, which is currently on its third beta. The update includes new iPhone XR features, new Apple Watch complications, and more.