As far as docs phrasing goes, it probably makes sense to frame it as "text" being the preferred argument name in 3.7+, with "universal_newlines" retained indefinitely as a compatibility preserving alias. After all, if that wasn't our intention, we wouldn't be adding the more straightforward alias in the first place.

As a new Py3-only argument, "text" can also be made keyword-only. (The Popen arg list is so long that most folks treat everything other than the first item as keyword-only anyway, but it doesn't hurt to ask the interpreter to enforce that)