This behavior still exists in 3.7.3:

Python 3.7.3 (default, May  1 2019, 00:00:47) 
[Clang 10.0.1 (clang-1001.0.46.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from dummy_threading import Thread
>>> def f(): print('foo')
... 
>>> t = Thread(target=f)
>>> t.start()
foo
>>> t.is_alive()
True
>>> t.join()
>>> t.is_alive()
False


It is inconsistent with the threading module behavior (which matches the 2.x behavior):


Python 3.7.3 (default, May  1 2019, 00:00:47) 
[Clang 10.0.1 (clang-1001.0.46.4)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from threading import Thread
>>> def f(): print('foo')
... 
>>> t = Thread(target=f)
>>> t.start()
foo
>>> t.is_alive()
False
>>> t.join()
>>> t.is_alive()
False

I would classify this as a bug since the documentation claims the dummy_threading module is supposed to be a drop-in replacement for the threading module.