> I'm sorry, but this seems like it should be an importlib internal
> affair.  The new exception is too much in everyone's face, because
> the exception name gets printed on every traceback.

That's the crux of the issue.  If there isn't much utility outside importlib to distinguishing between module-not-found and other causes of ImportError, then there isn't much point to a new exception.  It just boils down to what the other potential causes of ImportError are and how much people care about them.

I keep thinking about PEP 3151 (IOError/OSError hierarchy rework) and the lessons we've learned about exception attributes vs. subclasses.  For readability and write-ability, I'd rather write this:

  try:
      from _collections import OrderedDict
  except ModuleNotFoundError:
      pass

than this:

  try:
      from _collections import OrderedDict
  except ImportError as e:
      if e.reason is not importlib.machinery.ImportReason.MODULE_NOT_FOUND:
          raise

But the relevant question is, what is the benefit (outside importlib) of either over this:

  try:
      from _collections import OrderedDict
  except ImportError:
      pass