Now that Swift 2.2 / iOS 9.3 / watchOS 2.2 is out, can I update an app using Xcode 7.3 and still work on a iOS 9.2 device?
For example, if I started using #selector(someFunc) instead of selector("someFunc"), will this crash on a iOS 9.2 device?
iOS SDK is maintained to be backward-compatible (though I don't know, how far back). You can take a look at page SDK Compatibility Guide, Base SDK and Deployment Target Settings to read about "Base SDK" and "Deployment Target" project settings.
The latest language version (either Swift or Objective-C) can also be used with older iOS versions, because, no matter what the language is, the compiler will generate effectively the same machine-code at the end. There could be nuances like lack of some OS features (e.g. ARC is fully supported starting from iOS 5, though it's pretty much compiler/language feature).
It may be also interesting to read Xcode 7.3 Release Notes to get the list of new features and changes. This document does not states that some older iOS versions are no more supported.
