It appears this has shipped with the Xcode 13.2 beta.
- You can now use Swift Concurrency in applications that deploy to macOS 10.15, iOS 13, tvOS 13, and watchOS 6 or newer. This support includes async/await, actors, global actors, structured concurrency, and the task APIs. (70738378)
One question I have is whether there's a way to properly conditionalize concurrency code so that it can build in Xcode 13.0/.1 and 13.2 at the same time. I can't think of one offhand.