iOS 11 is part of a radical, but expected, decision taken by the Apple company, the Americans acting exactly as they do every year, after launching a new major version of iOS. More specifically, iOS 11 was completely blocked from being installed on iPhone, iPad, and iPod Touch by Apple, something that probably everyone knew would happen.
iOS 11 has been blocked for installation since last night, three weeks after iOS 12 was released by Apple, this being Apple's "standard" operating procedure. Blocking the possibility to install iOS 11 comes after Apple launched iOS 12.0.1 during the last night, the new update solving a series of iOS 12 performance problems.
iOS 11: The RADICAL decision taken by Apple
iOS 11 is no longer signed by the Apple company for installation on iPhone, iPad, and iPod Touch, so no downgrade procedure allows its installation. If you have not yet updated to iOS 12 because you are satisfied with iOS 11, you should be very careful not to make any mistakes that could force you to restore.
iOS 11 also has various jailbreak solutions available, and for the latest versions of the operating system others may appear, so don't rush to update. Having said that, from now on iOS 12 and iOS 12.0.1 are the only versions of iOS that can be officially installed, the first will be blocked from installation in about two weeks.
