I don't think using repr(self) as a fallback was intentional, that is why I wrote regression, but I may be wrong.

I agree that the previous default 'Thread-x' gives little information, but only the user can give a relevant name to describe what a thread is doing.
Using a new default like "ThreadPoolExecutor-worker_x" would give more information but at the same time it leaks internal information about how the thread was created in the code (which is not relevant when using the logging module), not about what it's doing.

Anyway as long as that repr string is replaced, I am happy :)