Thanks for bringing attention to this, Julien. 

While the regression is definitely unfortunate, I'm uncertain about whether the behavior is *correct* to allow in the first place. Specifically, allowing the registration of an atexit hook which uses a ThreadPoolExecutor within it means that the finalization of the executor will be done *after* thread finalization occurs, leaving dangling threads which will have to be abruptly killed upon interpreter exit instead of being safely joined. From my perspective at least, this doesn't seem like something to be encouraged.

Is there a real-world situation where it's specifically necessary or even beneficial to utilize ThreadPoolExecutor at this point after thread finalization rather than earlier in the program? Not that it doesn't exist, but to me it intuitively seems very odd to utilize an executor within an atexit hook, which are intended to just be resource finalization/cleanup functions called at interpreter shutdown. Assuming there is a genuine use case I'm not seeing, it may be worth weighing against the decision to convert the executors to not use daemon threads, as I presently don't think there's a way to (safely) allow that behavior without reverting back to using daemon threads.

That said, I'll admit that I'm more than a bit biased as the author of the commit which introduced the regression, so I'll CC Antoine Pitrou (active expert for threading and concurrent.futures) to help make the final decision.