ヒエラルキービューで選択しているシーン以外を一括でオープン／クローズするエディタ拡張。

ContextMenuSceneLoadUnload.cs

namespace Emptybraces.Editor
{
	using UnityEngine;
	using UnityEditor;
	using UnityEngine.SceneManagement;
	using UnityEditor.SceneManagement;
	// Editor extension that toggles the open/close state of all scenes except the one currently selected in the Hierarchy view.
	internal static class ContextMenuSceneLoadUnload
	{
		[InitializeOnLoadMethod]
		static void Init()
		{
			SceneHierarchyHooks.addItemsToSceneHeaderContextMenu += __Menu;
			void __Menu(GenericMenu genericMenu, Scene scene)
			{
				genericMenu.AddSeparator("");
				genericMenu.AddItem(new GUIContent("Open All"), false, () => OpenAllExcept(scene, false));
				genericMenu.AddItem(new GUIContent("Open All Except"), false, () => OpenAllExcept(scene, true));
				genericMenu.AddItem(new GUIContent("Close All Except"), false, () => CloseAllExcept(scene));
			}
		}

		static void OpenAllExcept(Scene exceptScene, bool isUnload)
		{
			for (int i = 0; i < SceneManager.sceneCount; ++i)
			{
				var scene = SceneManager.GetSceneAt(i);
				if (!scene.isLoaded && scene != exceptScene)
				{
					EditorSceneManager.OpenScene(scene.path, OpenSceneMode.Additive);
				}
			}
			if (exceptScene.isLoaded && isUnload)
			{
				if (exceptScene.isDirty && EditorSceneManager.SaveCurrentModifiedScenesIfUserWantsTo())
					EditorSceneManager.CloseScene(exceptScene, false);
			}
			else if (!exceptScene.isLoaded && !isUnload)
				EditorSceneManager.OpenScene(exceptScene.path, OpenSceneMode.Additive);
		}
		static void CloseAllExcept(Scene exceptScene)
		{
			if (!exceptScene.isLoaded)
				EditorSceneManager.OpenScene(exceptScene.path, OpenSceneMode.Additive);
			for (int i = 0; i < SceneManager.sceneCount; ++i)
			{
				var scene = SceneManager.GetSceneAt(i);
				if (scene.isLoaded && scene != exceptScene)
				{
					if (scene.isDirty && !EditorSceneManager.SaveCurrentModifiedScenesIfUserWantsTo())
						return;
					EditorSceneManager.CloseScene(scene, false);
				}
			}
		}
	}
}

ContextMenuSceneLoadUnload.md