Apple has stopped signing iOS 10.3.3 and iOS 11, making downgrades to pre-iOS 11 firmware impossible. The latest iOS version currently available to the general public is iOS 11.0.2.
Ios 11.0.2 RSS
Apple has released iOS 11.0.2, with a fix for the iPhone 8 crackling earpiece issue. The update also tackles the issue of attachments in S/MIME encrypted emails not opening, among other fixes.