Something like GitHub - amical/discourse-watch-category: Watches a category for all the users in a particular group · GitHub will do that. (You’ll need to customize it for your site; there is no UX for the plugin).
That’s already user selectable too, though I think with the categories on/off you might not need it. You could probably add changing that to the plugin as well.
If you’re hosted, you’d need to be on a plan that allows such plugins.