Which other IRC client implemented that? There's no notification of when someone sets themelves away or back so the only way for a client to tell is to continuously 'ask' the server using /who requests. Besides being messy, it'd also be a very considerable -and most likely unwelcome- drain on any IRC server if 95% of the users suddenly stat sending /who requests for every channel they're on every few seconds.
The only way that such a thing would become feasible is if it were implemented on the server side via some kind of notification of changes in a user's status.
