On Tuesday, Apple pulled the plug on signing iOS 9.3.5 for its entire iOS device lineup, and since it was the last iOS 9 release since iOS 10’s launch last month, that means you can’t downgrade your firmware anymore.
It’s worth also mentioning that the signing process was also killed for iOS 10.0.1, which means anyone who tries to restore their device(s) in iTunes will be forced to install iOS 10.0.2 or later (10.0.3 is only for iPhone 7 & 7 Plus).