If you meant "you have to necessarily flatten", check 3D displays like the ones based on screen rotation,

see https://news.ycombinator.com/item?id=49780526 for example: it is a DIY, and there is a demo of playing Doom in apparent 3D.

Another system in which matching interactive design and art could be difficult - but surely not impossible, and, on the contrary, a good challenge.

Some 2D games are played in 3D space.