Apple today has stopped signing iOS 11.3. Following the release of a new version of iOS, Apple routinely stops signing older ones to ensure the highest numbers of users are up to date.
With Apple no longer signing iOS 11.3, this means that users cannot downgrade from iOS 11.3.1 for any reason.
Apple regularly stops signing code in an effort to prevent users from installing old iOS versions. In addition to system security, restricted code signing helps ensure users have the most up-to-date firmware running on their iPhone, iPad and iPod.
The company is currently beta testing iOS 11.4, which promises AirPlay 2 support, Messages in the Cloud integration and Apple’s new ClassKit framework.