Gregory's Server
The answer, which I'm sure you're dying to know, is that the current version of Xcode and Swift 5.9 can compile an app for iOS 7.
The major caveat being no armv7 support — which means you can only build for 64-bit, so the only possible device this could ever run on is a non-upgraded iPhone 5s.
You also have to substitute in a bunch of arclite libraries into the SDK, which are trivial to find if you go looking
I know Swift can't compile for iOS 6 as I've gone down that road in the past — you could, at one point, force it to happen, but there were all kinds of ABI issues and even slightly complex code would segfault. iOS 6 was never officially supported by Swift (though the betas of Swift 1.0 did work, unofficially). But it's kinda nice to know that modern Swift is still back-compatible all the way to iOS 7.0, the first OS Swift supported