I have looked far and wide for the cause of the GCD hang. It was Xcode all along. queueDebuggingEnabled = "No" should be the default choice. That option should be off unless debugging a dispatched work issue. It really hurts performance. It can eventually cause the GCD kernel thread explosion issue without any reasonable way to debug it with Xcode or Instruments.
After scouring documentation, rewatching all of the WWDC sessions, attempting to guess solutions, and crawling the forums, I greatly appreciate this answer as I have truly sunk a great deal of time trying to appease the GCD. I realize the feature is super cool that it works, but it has not been useful for debugging any of my applications as you typically only have a couple DispatchQueues to juggle.
Here is an appropriate reproduction: Use CoreBluetooth with Combine to plot data in real-time with Swift Charts without blocking the main thread. Run for hours, it will happen as long as you are dispatching on and off the main queue. It may only take an hour before it is unusable if your notification rate is high enough. In my experience with several of the frameworks, a real-world bluetooth streaming application would be of great benefit to the acceptance testing suite. There are several long existing hangs and degradations that would be exercised by including such a test in the standard testing procedures.
Once again thanks for your accurate answer Eskimo!
