GrayScaleNode.cs

using System.Reflection;
using UnityEditor.ShaderGraph;
using UnityEngine;

[Title("Custom", "Gray Scale Node")]
public class GrayScaleNode : CodeFunctionNode
{
    public GrayScaleNode()
    {
        name = "Gray Scale";
    }
    
    protected override MethodInfo GetFunctionToConvert()
    {
        return GetType().GetMethod("GrayScaleFunction",
            BindingFlags.Static | BindingFlags.NonPublic);
    }

    static string GrayScaleFunction(
        [Slot(2, Binding.MeshUV0)] Vector2 UV,
        [Slot(3, Binding.None)] SamplerState SS,
        [Slot(4, Binding.None)] out Vector4 RGBA
    )
    {
        RGBA = Vector4.zero;
        return @"
                {       
                    const float3 W = float3( 0.2125, 0.7154, 0.0721 );
                    float3 irgb = _MainTex.Sample( SS, UV ).rgb;
                    float luminance = dot( irgb, W );
                    RGBA = float4(luminance,luminance,luminance,1.0);
                }
                ";
    }
}