Non-planar quads are normal, smooth shading hides the edges between them. It’s severely non-planar quads that cause problems, which are usually caused by bad edge/polygon flow where the structure doesn’t follow the forms. In the example case more geometry was needed to lessen the angle between polygons so that the shading can be more effective.
There are exceptions
- Even perfectly planar and convex quads can cause problems with Cycles. It suffers from shadow artifacts with low poly models, which is also known as shadow terminator problem
- When triangulation can’t change between applications, you have to triangulate the mesh with actual edges to make sure that doesn’t happen. One such case is when baking normals for an external application, triangulate before baking.