I don't even know what the offline mode is supposed to do, since it already asks me if I want to log in or not in a separate dialogue.
But FWIW: I have Steam version and the Offline button toggle does exist on my main screen.
However, playing with it just now:
If I do log in then click offline mode, the icon changes to offline (works?).
If I don't log in then click offline mode, I get the following error:
"Can't enter offline mode: error 502."
<Shrugs>