There were other exceptions of who could and who couldn’t show a little cleavage. The upper class could while wearing evening gowns, the French, sometimes the upper class did at times that weren’t at balls or dinner parties. the working class and sensible women didn’t. On the other end of the extreme, loose women and scandalous women could get away with it. And who was more scandalous than the French? (I’m part French by the way, so I am completely fine with that!)
If you are being historically accurate and you want to wear a “corset” outside the clothes, you would actually be wearing a bodice or waist cincher. Those go outside the clothes.