Adding support for GPU instancing for legacy shaders?

I'm using an Instanced Diffuse Shader. It's a great style.

Note that Legacy -> Diffuse is essentially a Surface Shader using the "Lambert" lighting scheme. Using that knowledge we can recreate the Diffuse Shader and add instancing.

Right click in your project window, go to Create -> Standard Surface Shader (Instanced).

Now you can tweak the Surface Shader to be diffuse by following this example here: https://docs.unity3d.com/Manual/SL-SurfaceShaderLightingExamples.html

I'll help you as well.

In the shader change the "#pragma surface surf Standard fullforwardshadows addshadow" to "#pragma surface surf Lambert fullforwardshadows addshadow"

Now change the "surf" function to

     void surf (Input IN, inout SurfaceOutput o) {
         fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * UNITY_ACCESS_INSTANCED_PROP(_Color);
         o.Albedo = c.rgb;
         o.Alpha = c.a;
     }

And delete all the references to "Metallic" and "Glossiness". You can delete the MainTex as well if you aren't using texturing.

BOOM! You have an instanced diffuse shader!