Apple has stopped signing iOS 10.3.3 and iOS 11.0, which means that Apple users will no longer be able to downgrade to pre-iOS 11 firmware. iOS 11 was released last month, and has undergone two iterative bug fixing updates, iOS 11.0.1 and iOS 11.0.2, since then.
Consumers who own an Apple device, which has been upgraded to any version of iOS 11, will be unable to downgrade to a pre-iOS 11 version. However, a user can downgrade from iOS 11.0.2 to 11.0.1 if he or she wishes to do so. In short, if you"re on an earlier version of iOS, make sure not to accidentally upgrade just to check out the new features, since there will be no way for you to downgrade after that.
iOS 11 brought a huge amount of changes to both iPhone and iPad, including an all-new App Store, a new notification center, storage optimization, and more. In addition, iOS 11 security updates and changes were introduced as well. Since then, two updates, primarily concerned with big fixes, have been pushed out to further enhance user experience.
Developers can currently download iOS 11.1, which is not available to the general public at the moment, but will possibly be released in the near future. It is important to note that Apple not signing older versions of iOS after new releases is fairly routine, as the company reportedly lays significance upon users keeping their operating systems up-to-date.
Source: MacRumors