Yes... unplugged "hey Siri" is a 6S/S+ only feature... however - lesser devices now have an ability to "train" Siri to their voice so there will be less of a chance of random activations of the feature when plugged in.
Have a ipad mini with 128gb of int storage, just upgraded to IOS 9. No hey siri without unplugged to battery?? That just sucks. Why, why, why??? Thought for some reason it was good on the ipad.
Vinny
PS My iPhone 6S+ is on its way.
I recently spent a LOT to purchase a 6 plus. Only allowing a 6 plus to connect with Siri unplugged seems unjustified. The 6 plus should be able to handle this just fine. It seems like a bald attempt to force upgrades.
Well, this actually irritated me, and for the first time today I came up with a solution. Instead of Siri, I can now, when unplugged or plugged, say, "Hey Google" and find out what i need to know.