It went ABI around Swift 5 or so.
I took a huge leap of faith, in 2014, and started using Swift, exclusively. It's turned out OK.
I'm not quite as positive about SwiftUI, though. I think it will work out, but it has a huge amount of catching up to do, if it is to replace UIKit/AppKit.
I'd be quite interested in Rich Siegel's take (the guy behind BBEdit). He's been at it, a while, as well.