Apple in the introduction of the new system version, it will turn off the old version of the system validation, and was shut down the system validation version means that no longer can not brush back to the original version, and only the latest version.
In contrast, Apple's official explanation is that in order to prevent the device from being downgraded to a software version that lacks the latest security updates, iOS uses a process called System Software Licensing. If you allow downgrade, an attacker who can physically access the device will be able to install an earlier version of iOS and take advantage of the security vulnerabilities that have been fixed in the new version of iOS.
In terms of popularity, Apple does not allow equipment degradation is out of security considerations.
Yesterday Apple closed iOS 10.3.3 and iOS 11.0 system authentication channel, which means that has been upgraded to iOS 11 iPhone, iPad and iPod Touch users will not be able to return to the original system.
Surprisingly, today Apple re-opened the iOS 10.3.3 system authentication channel, but it is limited to iPhone 6s models, you want to downgrade the fruit powder can try brush machine price.