Following the release of iOS 11.4.1 earlier in July, Apple on Tuesday stopped signing code for the prior iOS version 11.4 to ensure users are running the latest operating system on their devices.

Apple typically ceases code signing for legacy iOS builds roughly one month following the release of a newer version, but in this case, the company halted installations of iOS 11.4 only a week after iOS 11.4.1 was pushed out. 

The latest iOS 11.4.1 update addresses a number of bugs and patches a few critical security holes. Among the batch of changes was a fix to problematic code that caused some devices to crash when users typed the word "Taiwan" or received a text that contained the Taiwanese flag emoji.

In addition, Apple instated USB Restricted Mode as an option for iPhone and iPad owners. The security feature is designed to disable hardwired USB data connections after a preset period of time in a bid to protect user information from brute force passcode attacks.

The latest iOS revision also fixes AirPod tracking in Find My iPhone and improves the reliability of mail, contact and note sync for Microsoft Exchange. Prior to the update, users complained of battery drain issues that appeared to stem from background data syncing. 

Apple regularly ceases code signing to prevent users from installing old, potentially buggy iOS versions following the release of a new OS. The strategy keeps users safe and ensures devices are running the most up-to-date software, thus easing potential compatibility issues with new features and software. 

We failed to flash iOS 11.4 an iPhone 6s in 3uTools, you can't downgrade to it anymore, users can only download and install iOS 11.4.1

Source: appleinsider