Note: These events do not trigger for any other types of windows. Channel windows are handled by the on JOIN/PART events.
That's fine, but isn't related to actually closing the window if it's open while disconnected.
If you mean you want to CLOSE the window when you're disconnected, then I'd agree. However, from what I understand, this person wants to be able to check when the channel is being closed (or parted such as /partall) while offline. See how it relates now?
Since "on part" doesn't trigger while you are offline (and since on close doesn't work for channels -- since on part is supposed to handle it), there is no way to do anything with the channel buffer when closing the window while offline.
Checking if a channel window is open or closed isn't a big issue. Timers can do that easily enough. But the people that want this event, generally want it for more than just saying "oh hey this window is now closed."