I recently began developing a Mac app in Swift 3.0 using the Xcode 8 beta. I plan to finish development by the middle of August which would be before the public release of macOS. My current development machine runs El Capitan and my compiled Swift 3.0 code runs well on the current operating system version, which is a good sign, but I'm slightly concerned that there may be some issues running the code on previous OS X versions (e.g. 10.10). Is this a valid concern? In addition, will I even be able to release the app with Swift 3.0 before macOS is released?
Every Swift app is shipped with the Swift standard library included. The advantage of this is that it can run on multiple OS versions by default. The negative point is that it adds a few MB to your final app bundle.
So yes, your app will work on previous versions but keep in mind that you can't submit apps to the Mac App Store until Xcode 8 GM will be released.