> The commit for this bug (720f0cf580e2) introduces encoding and errors arguments but doesn't actually document what the values of these should be

Do you mean it doesn't document that they take the normal encoding/errors arguments that all the other functions that do character encoding take? Or they don't specify which encoding you should use? The latter is not feasible, and if you mean the former then feel free to open a new issue for documentation.

Having a single "text" parameter is essentially what we had before - a magic option that modifies your data without giving you any ability to do it correctly. This change brought Popen in line with all the other places where we translate OS-level bytes into Python str, and it's done it in exactly the same way.

Also see msg274562 where Nick withdraws his own suggestion of a "text" parameter.