http://wccftech.com/apple-stops-signing-ios-10-3-3-you-can-no-longer-downgrade-ios-11-to-ios-10/

This is extremely bad news for some users – Apple has stopped signing iOS 10.3.3 for iPhone and iPad. This means you can no longer downgrade iOS 11 to iOS 10 on your device.

With the release of iOS 11 done and dusted, it was only a matter of time Apple would stop signing iOS 10.3.3. The day has finally come and the inevitable has happened. This means that if you were hoping to downgrade your device back to iOS 10 (for whatever reason) you can’t do so anymore no matter what you try. In fact, if you downloaded the iOS 10.3.3 firmware file onto your computer and restored using iTunes, the process would simply fail. So please, rather than try it out yourself, it’s best you don’t tinker with your device at all. If you have updated to iOS 11, iOS 11.0.1 or iOS 11.0.2, then that’s the firmware you have to stick around with for a while until or unless a miracle is bestowed upon us from the iOS community. Also, keep in mind that Apple has also stopped signing iOS 11.0, the initial release of the firmware, apart from iOS 10.3.3.