Question: In WWDC 2015, they say that we shall use 1x,2x,3x images in assets because iphone 6p will only download 3x image and ignore other. I have watched about App Thinning. 

I would like to know whether it is only for ios 9 or below also?

Answer: 
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> Note: For iOS apps, sliced apps are supported on the latest iTunes and on devices running iOS 9.0 and later; otherwise, the App Store delivers universal apps to customers.
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From [App Distribution Guide - App Thinning](https://developer.apple.com/library/prerelease/watchos/documentation/IDEs/Conceptual/AppDistributionGuide/AppThinning/AppThinning.html)
Its for all the version of the ios. In the below version also you need to use the same structure of the images in the app. So that the device will select the required image from the image asset.