"build the concurrency library for back deployment'" - what does this merged request mean in practice?

[Diggory]

Swift concurrency back deployment

So @Douglas_Gregor 's PR to allow the concurrency library for back deployment was merged but it's not clear to me how this will actually be used. This looks to me like a build option that has to be specified when building Swift, and not something that could be used by a developer using Swift. Is there any news on whether concurrency will in fact be back deployed to older OSes? NOTE: I would like to request that this thread just discuss the technical details involved, and not descend …