The all mighty ReadOnlyAttribute

ReadOnlyAttribute.cs

using UnityEngine;

///<summary> The ReadOnly attribute for fields, allows inheritance and have its own custom drawer </summary>
public class ReadOnlyAttribute : PropertyAttribute
{}

ReadOnlyDrawer.cs

// -----------------
// Resources
// https://docs.unity3d.com/ScriptReference/MessageType.html
// https://discussions.unity.com/t/how-to-make-a-readonly-property-in-inspector/75448/7
// https://docs.unity3d.com/ScriptReference/GUI-enabled.html
// -----------------
using UnityEditor;
using UnityEngine;

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

        EditorGUI.PropertyField(position, property, label, true); // Include children could be the attribute's property

        GUI.enabled = true;
    }
}