I think it may be a good thing for there to be some sort of very obvious live alert/popup when a topic is closed.
For tl4 and staff it’s super easy to miss that a topic got closed while they were in it and they can unintentionally post through the close.
For tl3 and below, there’s no indication the topic got closed if they have the composer open and are writing a post - they don’t get informed until AFTER they hit the reply button when they’re done writing. (Potentially wasting a bunch of time writing when they could have just stopped and left on close).