Working code exists: A Persistent Vector, Persistent Hash Set, and Persistent Hash Map, all with COW support.
COW makes persistent collections particularly nice to use in Swift because support for a mutating API is possible in a safe and sane way without creating shared mutable state.
Plus, the mutated copies of persistent collections in case a collection is shared before local mutation are extremely cheap. In the non-shared case, direct manipulation is performed, just as in the case of the built in collection types.
The concurrency tag was added because of the usefulness of persistent collections for shared immutable state without a performance penalty in the case of local mutation.