Having no interest whatsoever in moving to Sierra I was startled to discover that Swift 3 requires Sierra. Worse, Apple has hidden the Swift 2.3 documentation — if it even still exists — so well that after a couple of hours searching and surfing I couldn’t find any trace of it. Except: The documentation in Xcode 8.2 on El Capitan seems to still be Swift 2.3, because Foundation classes begin with “NS”, which is no longer true in Swift 3. (How‘s that for backward compatibility! Though they do provide a migration tool.) I haven’t studied the Xcode 8.2 Swift documentation well enough to understand which version it is really documenting, though.
Playgrounds are affected to: “Playgrounds in Xcode  only work with Swift 3,” Swift 3 and Xcode 8 - Apple Developer
If anyone knows where to find Swift 2.3 documentation outside of Xcode, I would desperately like to know that.