In reality, there is no such thing as 2D.
In fact, if you want to make a game that supports ray tracing, dynamic, or static lighting, every 3D object you have should have an inside and an outside and be able to “hold water” inside it without leaking. The idea is that you never want light or a raycast of any kind to hit the back of a face. The normal of every face on a model should be pointing out, never in.