Following the release of iOS 12.0.1, Apple has stopped signing iOS 11.4.1 — the latest release of iOS 11. This means that it is no longer possible for iPhone X and older users to downgrade their device back to it.
An average consumer is going to care little about this change especially since iOS 12 brings plenty of new features and notable performance improvements even for older devices. However, jailbreakers usually like to stay on an older release of iOS as there’s a higher possibility of a jailbreak being available for it. Yalu jailbreak developer Luca Todesco has already managed to jailbreak the iPhone XS Max running iOS 12, though it is unlikely that he will release the jailbreak to the public.
If you have not yet upgraded your iPhone to iOS 12 and are still running iOS 11.4.1 on it, you should now be more careful of not upgrading to iOS 12. This is because if you do there’s no way you can downgrade back to iOS 11.4.1 now.
Apple is currently only signing iOS 12 so users who have upgraded to iOS 12.0.1 can downgrade back to it. However, given that iOS 12.0.1 brings about some major bug fixes, I doubt anyone would want to downgrade from it.