this will set a shader keyword and make sure it's only true in editor and never in build by disabling before the build and stripping and shader variants that have that keyword

UnityEditorKeyword.cs

// to use this define 
// #pragma shader_feature UNITY_EDITOR
// in your shader

#if UNITY_EDITOR

using System.Collections.Generic;
using UnityEditor;
using UnityEditor.Build;
using UnityEditor.Callbacks;
using UnityEditor.Rendering;
using UnityEngine;
using UnityEngine.Rendering;
#if VRC_SDK_VRCSDK3
using VRC.SDKBase.Editor.BuildPipeline;
#endif
namespace Shaders.UNITY_EDITOR_TEST.Editor
{
    [InitializeOnLoad]
    public class SetKeyword
    {
        static SetKeyword()
        {
            Shader.EnableKeyword("UNITY_EDITOR");
        }

        [PostProcessBuild(1)]
        public static void OnPostprocessBuild(BuildTarget target, string pathToBuiltProject)
        {
            Shader.EnableKeyword("UNITY_EDITOR");
        }

    }

    #region Callbacks

#if VRC_SDK_VRCSDK3
    public class OnBuildRequest : IVRCSDKBuildRequestedCallback
    {
        public int callbackOrder => 3;


        public bool OnBuildRequested(VRCSDKRequestedBuildType requestedBuildType)
        {
            Shader.DisableKeyword("UNITY_EDITOR");
            return true;
        }
    }
#if !UDON
    public class OnAvatarBuild : IVRCSDKPreprocessAvatarCallback
    {
        public int callbackOrder => 3;

        public bool OnPreprocessAvatar(GameObject avatarGameObject)
        {
            Shader.DisableKeyword("UNITY_EDITOR");
            return true;
        }
    }

    public class OnPostAvatarBuild : IVRCSDKPostprocessAvatarCallback
    {
        public int callbackOrder => 3;

        public void OnPostprocessAvatar()
        {
            Shader.EnableKeyword("UNITY_EDITOR");
        }
    }
#endif
#else
	public class OnBuild : IPreprocessBuildWithReport
	{
		public int callbackOrder => 3;

		public void OnPreprocessBuild(BuildReport report)
		{
            Shader.DisableKeyword("UNITY_EDITOR");
        }
    }
#endif

    #endregion

    public class ShaderPreprocessor : IPreprocessShaders
    {
        public int callbackOrder => 3;

        public void OnProcessShader(Shader shader, ShaderSnippetData snippet, IList<ShaderCompilerData> data)
        {
            for (var i = data.Count - 1; i >= 0; --i)
                if (data[i].shaderKeywordSet.IsEnabled(new ShaderKeyword("UNITY_EDITOR")))
                    data.RemoveAt(i);
        }
    }
}

#endif