suggests that it was previously available but not on those platforms. You're going to find a lot of people searching high and low for the .exe file IMO.
Personally I'd stick with "Available on [apple]/[linux]", drop the "only" if you're worried about it sounding like it's a choice for exclusivity reasons. Then you probably need to add a by-line below the button indicating "MS Windows and other OS versions not yet available, please contact us through the forum [or whatever] for details". Or have a click through to gather email addresses so you can contact users later if MS Win or other versions come out.
Don't try to butter me up. We all know you are still angling for cyan with Wilma from accounts. Well that dream is dead my friend, the CFO was spotted playing golf and all his clubs have turquoise handles.
