Presumably when he says 'At least with mIRC...' he means that mIRC's popularity makes it stand out as perhaps the only client which cannot implement the /who method since no other client's userbase would be large enough to create an issue. Of course it is still possible with mIRC using a simple script.
As for away notification in general, even a server-side implementation could be problematic as a means of abuse. It would effectively give a single user the ability to send something to every user they share a channel with. That's the kind of thing flooders and DoS'ers dream of.
