Windows 3.1 UI was not flat design. And even things like System6 weren't that _flat_. There were subtleties on their UI that gave a little depth to some stuff.

Windows 3.1 UI was not flat design. And even things like System6 weren't that _flat_. There were subtleties on their UI that gave a little depth to some stuff. But one could argue the _flat_ in their UI was some sort of answer to the limitations of their time.