But that's not what it does. Try it.

    $ git status
    On branch master

    No commits yet

    Untracked files:
      (use "git add <file>..." to include in what will be committed)
     foo

    nothing added to commit but untracked files present (use "git add" to track)
    $ git status --porcelain=v1
    ?? foo
    $ git status --porcelain=v2
    ? foo
    $ git status --porcelain=v3
    fatal: unsupported porcelain version 'v3'

No --porcelain option gives you the output in the format of the git status porcelain, either at the current version or at Git 1.x or whatever, i.e., "give me the porcelain" is the wrong reading of the option.

The correct reading is "I am implementing a porcelain myself, give me a format I can parse in order to do that."