>Wouldn't First class concurrency in a way breaks ABI and Stdlib as well? ( Otherwise it would be second class )

Well, theoretically you can add new syntax without altering/affecting existing syntax and programs, and similarly for ABI.

From what I gathered from the email, first class concurrency won't be in swift 4 either way, just that they want to start talking about it in phase 2. Their separation of stage 1 (ABI-breaking changes) and stage 2 (non-ABI-breaking changes) was in regards to how they prioritize swift 4 tasks. (In other words, the discussion itself won't introduce any ABI-breaking changes, so there's no need to make sure it gets done in phase 1.)