Docs: threading says dummy thread objects are never deleted, outdated since 3.13 #159011

Documentation

Doc/library/threading.rst says about dummy thread objects:

They are never deleted, since it is impossible to detect the termination of alien threads.

Since 3.13, Lib/threading.py does detect it: _DeleteDummyThreadOnDel is stored
in a thread-local (_thread_local_info). When the alien thread finishes, the
object is collected and removes the dummy thread from threading._active.

import _thread, threading, time
done = threading.Event()
def f():
    threading.current_thread()   # creates a _DummyThread
    done.set()
_thread.start_new_thread(f, ())
done.wait(); time.sleep(0.3)
print([t.name for t in threading.enumerate()])

Suggested fix: drop "They are never deleted…" and say that since 3.13 a dummy
thread object is removed once the alien thread finishes. Add a versionchanged:: 3.13 note.

Linked PRs

• gh-159011: Update dummy thread lifecycle documentation #159032