When I read this, it rang a bell. I had a feeling I’d seen something related to it not too long ago. At first, I thought my sense of time was off and that it was the component Lilly linked. But I think I’ve finally found the PR I remembered reading. It was merged yesterday, so you may need to update before it’s available to you:
This change adds a hidden site setting named hide_disabled_create_topic_button. When the setting is enabled:
hides the “New Topic” button in read-only categories
hides the “New Topic” button when the current user doesn’t have create permissions
shows the “New Topic” button on other categories where the user has permission to post
I haven’t tried it yet, but it looks like this hidden site setting basically restores the old behavior of the New Topic button.