You can’t go home again, and you also can’t go back to iOS 9. Apple has stopped signing iOS 9.3.5 for the iPhone, iPad, and iPod touch, putting an end to iOS 10 to iOS 9 downgrades.
Apple also stopped signing iOS 10.0.1, making iOS 10.0.2 the oldest mobile operating system users can downgrade their device to via iTunes. This is no big scandal, as Apple routinely stops signing older versions of iOS as new ones are released.
In related news, they also stopped signing tvOS 9.2.2 for fourth-generation Apple TVs.
iOS 9.3.5 was released in late August, and was the last update for the iOS 9 operating system. The update was issued to address three zero-day exploits. iOS 10 hit devices on September 13.