iOS 12 has been released for several weeks now and if for whatever reason you’re thinking that maybe you would be better off using iOS 11, you might be disappointed to learn that will no longer be possible. This is because Apple has stopped signing copies of iOS 11.4.1 which was the latest version of iOS before iOS 12.
For those wondering what this means, a signed copy of an iOS build is basically Apple’s way of verifying/certifying the update. By no longer signing it they are saying that this is no longer a viable version of iOS, which means that you can’t use it anymore. Had this been a week earlier you could still technically downgrade.
As to why this might be a big deal? With previous releases of iOS, Apple had a bit of a QC problem where they seemed to allow bugs to pass through, and these are minor bugs with annoying side-effects that should have been caught. However so far iOS 12 seems to be relatively problem free, at least not on the scale of previous updates.
Not to mention the update also seems to show that Apple has kept to their word, which is improving performance on older devices, so there is actually more of a reason to update to IOS 12 if you’re running an older build of iOS.