You can achieve this flat shading effect, but it's quite limiting and in no way is cell/toon shading replacement. First you make sure that there's no other lighting in your scene except ambient environment, then set its colour to pure white and disable tone mapping in the VFB. Prepare the material as follows - disable reflection, plug the same colour map in base color (diffuse) and self-illumination slots, set self-illum multiplier to 1.0 Render.