You Can No Longer Downgrade to iOS 8
PSA: You can still downgrade to iOS 9.0.1 but you can no longer downgrade to iOS 8 (iOS 8.4.1) or iOS 9.0. Apple's stopped signing off on those updates and you're no longer able to drop back down to them if you encounter issues. Apple closed those loop holes up yesterday.
The company typically closes the loophole up about a week or two after an update's release so you'll want to keep an eye on iOS 9.0.1.
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