This is not entirely true. As discussed in this thread, open source Foundation still must match the API of Darwin Foundation (Even if it doesn't have parity with it). That thread dives into discussion about alternatives approaches, but the fact remains that Foundation has a closed evolution.
That being said, what exactly about URLSession or its capabilities would you want to discuss as part of the work group? Because from reading the new work group goals, it sounds like building a new Swift library that could do URLSessiony type things is on the table. (Or by putting an already existing library through the work group process)