Add to your scene to have a shader keyword enabled/disabled only for the scene view. Use for debug materials, scene visualisations

SceneViewShaderKeyword.cs

/* SceneViewShaderKeyword.cs
 * © Eddie Cameron 2020
 * ----------------------------
 */

using System;
using UnityEngine;

#if UNITY_EDITOR
using UnityEditor;
#endif

/// <summary>
/// Set a shader keyword when rendering the scene view in editor
/// adapted from https://forum.unity.com/threads/image-effect-in-scene-editor-views.102515/
/// </summary>
[ExecuteInEditMode]
public class SceneViewShaderKeyword : MonoBehaviour {
    public string keyword;
    public bool onInSceneView;

    #if UNITY_EDITOR
    Camera cam;
    SceneViewShaderKeyword sceneViewInstance;
    bool isSceneViewCam;
    bool previousKeywordState;

    private Camera GetCamera()
    {
        return SceneView.lastActiveSceneView?.camera;
    }

    // These are only called when attached to the scene view camera object
    void OnPreRender() {
        if ( !isSceneViewCam )
            return;

        previousKeywordState = Shader.IsKeywordEnabled( keyword );
        if ( onInSceneView )
            Shader.EnableKeyword( keyword );
        else
            Shader.DisableKeyword( keyword );
    }

    void OnPostRender() {
        if ( !isSceneViewCam )
            return;

        if ( previousKeywordState )
            Shader.EnableKeyword( keyword );
        else
            Shader.DisableKeyword( keyword );
    }

    void Update() {
        if ( !isSceneViewCam ) {
            // update scene view camera
            if ( sceneViewInstance == null ) {
                if ( cam == null )
                    cam = GetCamera();

                if ( cam == null ) // This shouldn't happen, but it does
                    return;

                sceneViewInstance = cam.GetComponent<SceneViewShaderKeyword>() ?? cam.gameObject.AddComponent<SceneViewShaderKeyword>();
            }

            sceneViewInstance.isSceneViewCam = true;
            sceneViewInstance.keyword = keyword;
            sceneViewInstance.onInSceneView = onInSceneView;
        }
    }
#endif
}