Following the release of iOS 12.0.1 earlier this month, Apple has now stopped signing iOS 12 for all compatible iPhones and iPads. This means users will not be able to downgrade back to iOS 12 on their iPhone or iPad for any reason whatsoever.
iOS 12.0.1 does bring a number of iPhone XS related bug fixes including improvements to network and Wi-Fi connectivity and wireless charging so most users are unlikely to downgrade back to iOS 12. That is unless they are holding out for a potential iOS 12 jailbreak. We have seen a few iOS 12 jailbreak demo but so far there has not been any public release of the same.
If your iPhone or iPad is running iOS 12, you now might want to be careful about not upgrading to iOS 12.0.1 since all gates of a possible downgrade back to iOS 12 are now closed.