Sorry, I mean “shader model” as in “graphics card flavor.”
(shader model 5 or 6; OpenGL versus Direct3D, etc.)
Also, if your shader model is set to “unlit” but you get that soft shading, then your shader is doing its own lighting computations somehow. At that point, whatever you do inside the shader will be the important bit, in addition to whatever limitations may come from the render/shader model.