Not (currently) supported before iOS 15
The big problem with the Swift 5.5 release on Apple platforms is that there is no support for the concurrency changes on OS versions before iOS 15 / Mac OS 12, which, despite the rapid upgrade of most Apple users, still leaves those real world developers whose apps support older OS versions out in the cold. It's fine for hobbyist developers, but in the real world forcing a latest OS minimum is tough - even if 90%+ of active devices will be using it within 6 months, that last few % tend to be *very* vocal.
There are some comments from internal Apple people on the Swift forums that work is ongoing to back port the changes but no certainty that it will work, or how far back it would offer support. A lot of major third party systems still support back as far as iOS 10.0, so it will be years before they move to a minimum of 15.0 and make this usable, in the meantime they are pushing forward with a Swift 6.0 which sounds like it might also need more OS support and thus be iOS 16 as a base.