Apple today stopped signing iOS 8.4.1 and the first version of iOS 9 for the iPhone, iPad, and iPod touch, meaning users can no longer upgrade or downgrade to those versions of iOS using iTunes. Apple is now signing iOS 9.0.1 and iOS 9.0.2 only.
Users who have upgraded to iOS 9 are now unable to downgrade back to the last version of iOS 8, iOS 8.4.1. Released in August, iOS 8.4.1 was a minor update that included bug fixes and Apple Music improvements. iOS 8.4.1 also broke the iOS 8.4 untethered jailbreak.