Example of how to have a context menu item in Unity that can only be used on folders.

UnityDirectoryContextMenuExample.cs

using System.IO;
using System.Linq;
using UnityEditor;
using UnityEngine;

public static class UnityDirectoryContextMenuExample 
{
    [MenuItem("Assets/My Context Item")]
    public static void DoSomething()
    {
        Debug.Log("Doing something.");
    }

    [MenuItem("Assets/My Context Item", true)]
    public static bool DoSomethingValidator()
    {
        var assetObject = Selection.GetFiltered<DefaultAsset>(SelectionMode.Assets).FirstOrDefault();

        if (assetObject == null)
        {
            return false;
        }

        return AssetIsDirectory(assetObject);
    }
    
    public static bool AssetIsDirectory(Object assetObject)
    {
        string path = AssetDatabase.GetAssetPath(assetObject);

        return File.GetAttributes(path).HasFlag(FileAttributes.Directory);
    }
}