delete/purging topics from the client side does not remove the category
You are right....so I am left with a 'fake' category which I disable anyway
If there are too many subcategories under a category, these categories are not displayed on the main page and on the categories page in the Admin panel. 10 categories under a category are displayed on the main page. More than 10 are not displayed.
You can increase that number in the settings of the category.