After the release of iOS 12.0.1 yesterday, Apple has stopped digitally signing iOS 11.4.1, the last update to the iOS 11 train. The result means that users will no longer be able to downgrade from iOS 12 to iOS 11.4.1 by downloaded the build from Apple’s update servers.
The change is normal for Apple. As a general rule, they stop signing the previous build of iOS about a month after the new build is released. This is to assure compatibility and overall security for iPhone and iPad owners. The knock-on effect is that, once you upgrade your iPhone or iPad to iOS 12.0.1, you will not be able to go back to iOS 11.4.1 as of today.
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 11. Given that the first bug fix release is now out, there is little reason to not upgrade your iPhone and iPad to iOS 12 at this point.
If you want to keep your iPhone or iPad on iOS 11.4.1, you can do so for now and simply not upgrade via the Software Update feature in iOS. You will continue to get notifications to update but you can ignore those for now.