ReadOnly.cs

using UnityEngine;
#if UNITY_EDITOR
using UnityEditor;
#endif

public class ReadOnlyAttribute : PropertyAttribute
{
}

#if UNITY_EDITOR
[CustomPropertyDrawer(typeof(ReadOnlyAttribute))]
public class ReadOnlyFieldDrawer : 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)
    {
        var guiEnabled = GUI.enabled;
        GUI.enabled = false;
        EditorGUI.PropertyField(position, property, label, true);
        GUI.enabled = guiEnabled;
    }
}
#endif

Usage example:

[SerializeField]
[ReadOnly]
private float m_somePrivateFloat;