We had the quirks of import related threading deadlocks documented for a long time, not as a promise, but as a debugging aid (and a recommendation for "don't do that").

I'd see this as being similar: we'd document that if you're using the _thread module, or otherwise allowing operating system threads not managed by the threading module to call in and run arbitrary Python code, then you should run "import threading" early in the actual main thread to make sure it gets associated correctly.