The answer is:
Honestly, I don't see how swift should be considered special here at all. It's certainly not any different from objective-c. And the only thing that perhaps separates these languages from other languages is that when you're writing code in these two languages, you're usually writing for a specific operating system (like you're almost always writing specifically for Windows when you're writing in c# or vb.net) (and unlike say java), but also there are two very common operating systems that are used.
So, a question in java doesn't typically need to specify an operating system, because it is rare that java questions are doing something OS-specific, and when they are doing OS-specific things, it's usually android, and gets tagged with that.
Meanwhile, up until recently, .net was enough to know that you were in the Windows environment. I don't know enough about it to know whether there are any real differences between developing for desktop or Windows phone... but there really aren't that many Windows phone specific questions anyway. When there are though, if there are real differences in development between the systems, an OS-specific tag should be used.
First, let's understand this aspect. Let's take a look at some stats... I'm not claiming these as perfectly accurate. I looked several places and they all report similar sorts of things... look at the popularity of various operating systems based on web traffic.
Per these particular stats, iOS 8 is the 3rd most popular operating system, and OS X is the 5th most popular operating system. Both are more popular than any flavor of Linux. Additionally, iOS 8 has less than double that of OS X.
But perhaps most importantly, there are drastic differences between developing between the operating systems.
UIKit.framework, which is use extraordinarily extensively through almost any iOS application isn't available at all for OS X.
- In iOS, you've got tons of restrictions on things like what part of the file system you have access to, which doesn't exist in OS X.
- In OS X, you can launch child processes and execute terminal commands and such--things you simply cannot do in iOS.
There are real and distinct differences between the operating systems that warrant the tag to provide the distinction.