There was a big thread about this question a few weeks ago. In that thread, Ted Kremenek explained:
Concurrency requires runtime support that does not backward deploy. The release notes imply that this is an "issue" that just needs to be fixed. It's not. It's a feature that would need to be implemented. At this time, folks should assume that concurrency does not backward deploy. That said, everyone is aware of the value of it doing so, and is something that is being explored/considered.
The reason it doesn’t backwards deploy is that parts of the concurrency feature are implemented in Grand Central Dispatch and other parts of the OS. (The surface-level concurrency APIs are also shipped in the OS, but we deliberately put them in a separate binary so we’d be ready to back-deploy them if the other technical barriers could be surmounted.) It’s not clear if concurrency can be cut loose from these changes or if the results would work well enough to recommend using. We’re exploring that, but for now you should assume the answer will be “no”.