What are you proposing? I think you will find both cases are true. The change was made in 3.4 (but only in time for 3.4.3+) and 3.5 (starting at 3.5.0). If the documentation failed to mention 3.5 and only mentioned 3.4.3, it would not be obvious if that change made it into 3.5.0, or only the upcoming 3.5.1, etc.

See the “Changed in version 3.3.1” notice at <https://docs.python.org/3/library/subprocess.html#subprocess.Popen> for a similar case, which mentions both versions.