Since the topic from which my quote originates is no longer public, I would like to briefly explain the context here.
It did not refer in any way to the creation of new categories, but only to the fact that unlisting an accidentally published topic in the #announcements category may not have the desired effect due to the configuration that all users are notified of new topics in the category by default and the fact that it is displayed to admins in “what’s new.”
Unlisting prevents the topic from being displayed in “latest,” but it does not make the topic private. Anyone who knows the link can open the topic, read it, and reply to it as in any other topic. Since many people had a link to this topic, I was concerned that the fact that anyone could read it and reply to it, but hardly anyone would see the replies of others, would harm the discussion in the topic.
Unlisting keeps a topic more hidden, but anyone with access to the category can access it and receives notifications just like for any other topic.