ReadOnlyAttribute.cs

## Put this file OUTSIDE Editor folder


using UnityEngine;
public class ReadOnlyAttribute : PropertyAttribute { }

ReadOnlyDrawer.cs

## Put this file INSIDE Editor folder


#if UNITY_EDITOR
using UnityEditor;
using UnityEngine;

[CustomPropertyDrawer(typeof(ReadOnlyAttribute))]
public class ReadOnlyDrawer : PropertyDrawer
{
    public override float GetPropertyHeight(SerializedProperty property, GUIContent label)
    {
        return EditorGUI.GetPropertyHeight(property, label, true);
    }

    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        GUI.enabled = false;
        EditorGUI.PropertyField(position, property, label, true);
        GUI.enabled = true;
    }
}
#endif

Example usage by adding [ReadOnly]

public class YourScript : MonoBehaviour
{
    [ReadOnly]
    [SerializeField]
    private int yourVariable;

    // Rest of your code
}