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.