How to reproduce:
1. Create a new project
2. Go to Edit -> Project Settings -> Script Execution Order
3. Click the '+' button to add a Script to the custom order

Expected result: read-only Scripts are not allowed to be added to the execution order and are not shown in the list
Actual result: read-only Scripts are not allowed to be added but are still shown in the list

Reproducible with: 2020.2.0a14, 2020.2.0b7, 2021.1.0a2
Not reproducible with: 2018.4.29f1, 2019.4.13f1, 2020.1.9f1, 2020.2.0a13

Notes:
1. the Scripts that are not allowed to be added belong to these namespaces: TMPro, UnityEngine.EventSystems, UnityEngine.UI, UnityEngine.Timeline, etc
2. If some Scripts are not allowed to be added to the execution order by design, then they should not be shown in the list