As is their customary process after a release of a new version of iOS, Apple has ceased signing iOS 12.0.1 on their update servers. That means that once you upgraded to iOS 12.1, it is all-but impossible for consumers to downgrade back to the previous version.
iOS 12.1 was released at the end of October. As a general rule, Apple stops signing a previous version of the platform two-to-four weeks after the new release. That fits within the scope of iOS 12.0.1 and 12.1.
For those new to iOS, all updates come from Apple for your iPhone and iPad, even if your device is locked to a carrier. When Apple stops “signing” a version of iOS, it essentially means that it is no longer available on their update servers and you can’t access it. For consumers, it means that once you update to the current release of iOS, the older version is no longer available.
Apple does this to keep customers up to date with the latest version of iOS but also to keep the supported versions down to a minimum.
If you are running iOS 12.0.1 on your iPhone or iPad and want to stay on it, make sure you don’t upgrade to iOS 12.1. Once you do, you can’t go back. To be fair, there are a lot of reason to upgrade to 12.1 including Group FaceTime, new emoji and other performance updates for your devices.