How can Xcode 7 Stack Views be backwards-compatible with iOS 8 and iOS 7?
UIStackView is supported from iOS 9 onwards (iOS9 and later).
Its availability has not been mentioned for its previous version till now.And its clearly said that its new feature for iOS9.
Hence the answer for your question is - its in not a backward-compatibility.
There is similar thing available (OAStackView) to support prior version of iOS.https://github.com/oarrabi/OAStackView