Following the release of the maintenance focused iOS 12.0.1 two weeks ago, Apple has stopped signing iOS 12.0 for devices. This means that users are no longer able to downgrade from 12.0.1 back to 12.0. For long time iPhone and iPad users, this news won’t come as a surprise as it fits within Apple’s normal process for software updates.
For those new to iPhone or iPad, when Apple stops signing a release on their update servers, it becomes no longer available. That, in simple terms, means that you are no longer able to go back to that build should you decide to reset you device. The Cupertino company usually does this about two weeks after each release of iOS. In the case of iOS 12.0, it was 14 days.
Ultimately this should have little impact on end users. The only users who are really impacted by this are those who have jailbroken their devices, an ever increasingly difficult proposition with iPhones and iOS. For “normal” consumers, the impact is the inability to downgrade back to iOS 12.0. Given that the first bug fix release is now out, there is little reason to not upgrade your iPhone and iPad to iOS 12.0.1 at this point.
If you want to stay on iOS 12.0 however, you can continue to ignore the upgrade warnings on your iPhone or iPad for now. Given that we are likely only a few days away from iOS 12.1 being released, the first major update to the iOS 12 training, you could consider waiting it out and getting that update when it is available.