I apologize.
They have had a Swift roadmap forever. I think it's now kept on swift.org. I'll see if I can find it. I think it's a fairly sparse one. I really only cared about the evolution through ABI Stable. All I needed to hear, was that was a goal.
You are right. They tend to eschew roadmaps, but they did a "hard-sell" with Swift. They knew it would be difficult to build momentum with.
"Full System API" is the native frameworks; UIKit, AppKit, WatchKit, etc., as well as things like WebKit and MapKit.
When Swift was announced, they had APIs for most of that stuff. I was pleasantly surprised. I had a full app, working within a day or so (using beta Xcode, of course).