I agree that the /category/ slug is not useful at all. Not useful from a Usability perspective and not useful from a SEO perspective.
But why not putting a URL value in the database for the categories to get rid of the id and the /category/ slug?
I agree, and to add to this: the /category/ and /topic/ slugs look a little bit weird on non-English installs of NodeBB. It would be nice if we can drop these for translation consistency.
I don't mind the number in the url; that is also helpful if two topics share the same name.
(Just wondering, is there an update for this request? ).