Personally, I’ve never had the need to strictly follow tags or categories but in reading this whole topic it does seem to be something quite important to some communities and I can definitely see the use case.
Perhaps some sort of site setting is needed here since this is not needed in all communities but its helpful to have. For example:
Enforce tag restrictions for staff
Or perhaps make the setting more granular in case you don’t want restrictions everywhere? In the category settings you would setup what tag group you want to enforce in the category, then you would choose what category you want to enforce in the site setting in the dropdown menu?
Enforce tags on specific categories for staff
Choose category