https://wccftech.com/apple-stops-signing-ios-11-4-downgrade-no-longer-possible-from-ios-11-4-1/

Apple released its latest iOS 11.4.1 firmware update to the public last week. Now, the company has stopped signing the previous iOS 11.4. There are several reasons why this might be a bad news for you, which we will talk about later on. Generally, with every new release of an iOS update, Apple usually stops signing the previous version as soon as it can. This is mainly to restrict users who have updated to the latest version from going back. So let’s dive in to see some more details on the matter and what jailbreak users should know.

As we have mentioned earlier, Apple has stopped signing iOS 11.4 following the release of iOS 11.4.1 last week. This means that if you have updated your iPhone or iPad to iOS 11.4.1, you can no longer downgrade to the previous version. As to begin with, there are several reasons why you would want to downgrade to the previous version. Mainly, a jailbreak is the reason why you would opt to do this.