I never ever used PYTHONTHREADDEBUG=1. I just tried:
---
$ PYTHONTHREADDEBUG=1 ./python -m test -v test_sys
PyThread_init_thread called
PyThread_allocate_lock() -> 0x1f212c0
PyThread_allocate_lock called
PyThread_allocate_lock() -> 0x1f21300
PyThread_allocate_lock called
PyThread_allocate_lock() -> 0x1f21340
PyThread_allocate_lock called
PyThread_allocate_lock() -> 0x1f3ea70
PyThread_acquire_lock_timed(0x1f212c0, -1, 0) called
PyThread_acquire_lock_timed(0x1f212c0, -1, 0) -> 1
PyThread_release_lock(0x1f212c0) called
PyThread_acquire_lock_timed(0x1f212c0, -1, 0) called
PyThread_acquire_lock_timed(0x1f212c0, -1, 0) -> 1
PyThread_release_lock(0x1f212c0) called
PyThread_acquire_lock_timed(0x1f21340, -1, 0) called
PyThread_acquire_lock_timed(0x1f21340, -1, 0) -> 1
PyThread_release_lock(0x1f21340) called
PyThread_acquire_lock_timed(0x1f21340, -1, 0) called
PyThread_acquire_lock_timed(0x1f21340, -1, 0) -> 1
PyThread_release_lock(0x1f21340) called
PyThread_acquire_lock_timed(0x1f21340, -1, 0) called
PyThread_acquire_lock_timed(0x1f21340, -1, 0) -> 1
PyThread_release_lock(0x1f21340) called
PyThread_acquire_lock_timed(0x1f21340, -1, 0) called
PyThread_acquire_lock_timed(0x1f21340, -1, 0) -> 1
PyThread_release_lock(0x1f21340) called
(...)
======================================================================
FAIL: test_changing_sys_stderr_and_removing_reference (test.test_sys.SizeofTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/python/main/Lib/test/test_sys.py", line 1525, in test_changing_sys_stderr_and_removing_reference
    self.assertEqual(out, b"")
     ^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError: b'PyThread_init_thread called\nPyThread_all[59813 chars]ed\n' != b''
(...)
PyThread_release_lock(0x2113660) called
PyThread_free_lock(0x2113660) called
PyThread_free_lock(0x7fc6e8001300) called
PyThread_acquire_lock_timed(0x1f212c0, -1, 0) called
PyThread_acquire_lock_timed(0x1f212c0, -1, 0) -> 1
PyThread_release_lock(0x1f212c0) called
PyThread_acquire_lock_timed(0x1f212c0, -1, 0) called
PyThread_acquire_lock_timed(0x1f212c0, -1, 0) -> 1
PyThread_release_lock(0x1f212c0) called
PyThread_free_lock(0x1f3ea70) called
PyThread_acquire_lock_timed(0x1f212c0, -1, 0) called
PyThread_acquire_lock_timed(0x1f212c0, -1, 0) -> 1
PyThread_release_lock(0x1f212c0) called
PyThread_free_lock(0x1f212c0) called
PyThread_free_lock(0x1f21300) called
PyThread_free_lock(0x1f21340) called
---

This debug mode produces so many logs that it looks basically useless :-(

See also discussion at: https://github.com/python/cpython/pull/24241