This may have been reported before, but I couldn’t find anything.
The reply button is appearing on closed posts sometimes. It isn’t always and as far as I know, it doesn’t do anything but is still there. The screenshot below shows I can reply even though it is closed.
That’s not a closed post, it’s an archived topic, FWIW.
Your forum is about 6 months behind so it might be good to update first and see if the issue is still there.
That’s not this issue in this case, the reply button was even visible for me without authentication when I found the above topic through a search engine and visited the forum for the first time.
But with that button you couldn’t open the composer as it was reported in this topic, could you?
I think there was always a reply button for logged-out users. As long as the user doesn’t log in, you cannot tell if they have the permission to reply. That affects closed and archived topics, but also topics in categories where only some groups are allowed to reply. Anons see a reply button in all these cases
It’s also visible on closed topics on Meta.