You have to jailbreak current OS, then you can downgrade to iOS 6. It'll be jailbroken, but it's just enabled ssh, you can use it without installing Cydia and everything should work as normal. It could be done only with iPhone 4S or iPad 2 and only if current iOS is jailbroken (at least that was the case year ago, I was able to downgrade from iOS 8 to iOS 6 on iPhone 4S). Keyword to Google: OdysseusOTA. Officially there's indeed no way to downgrade, only using vulnerabilities.

I've always spoken well of Apple's software update record but it comes with a nasty flipside in my experience: unlike an old x86 PC or Android device, once you update the OS, you're generally stuck. No downgrading and certainly no "slimmed down" builds or alternate OSes to breathe new life into physically intact but not-current hardware.