The only problem with the 3.7+ documentation is the note about 3.2. Several paragraphs under <https://docs.python.org/release/3.7.1/library/subprocess.html#subprocess.Popen>, it still says

“Changed in version 3.2: The default for ‘close_fds’ was changed from False to what is described above.”

However the description it refers to was removed in 3.6. IMO it would make sense to add the description to the note:

“Changed in version 3.2: The default for ‘close_fds’ was changed from False to True on Unix. On Windows the default was changed from False to True when stdin/stdout/stderr are None.”