Unlike you, we have a lot of deleted topics and unfortunately, it is not possible to change them individually 
We have to change the category in batches.
On the other hand, when the topic is removed, it is not possible to change the category, and this causes it to do nothing more. We want to delete these categories but we do not know what the system behaves in this situation.
I think the only way left is to hide the categories instead of deleting them. That means we change the access to categories so that only admins can see them