ReadOnly property drawer for Unity: add a [ReadOnly] attribute to a serialized field to make it show up as read only in the inspector

ReadOnlyAttribute.cs

// Put this outside of an 'Editor' folder.

using UnityEngine;

public class ReadOnlyAttribute : PropertyAttribute { }

ReadOnlyPropertyDrawer.cs

// Put this to an 'Editor' folder.

using UnityEngine;
using UnityEditor;

[CustomPropertyDrawer(typeof(ReadOnlyAttribute))]
public class ReadOnlyPropertyDrawer : PropertyDrawer
{
    public override void OnGUI (Rect position, SerializedProperty property, GUIContent label)
    {
        GUI.enabled = false;
        EditorGUI.PropertyField(position, property, label, true);
        GUI.enabled = true;
    }

    public override float GetPropertyHeight (SerializedProperty property, GUIContent label)
    {
        return EditorGUI.GetPropertyHeight(property, label, true);
    }
}

As @Yolo-Jerome wrote, it's not perfect.
However, for all other cases, it is very useful and I thank you for it! :)

Best regards.