I wonder if the combat bonus can be applied to the city-state units which are levied by its suzerain?
Whether it's valuable to attack city-states, imo, is not depend only on the warmongering penalty, but also how easily you can earn the bonus of city-states.
In civ V, by completing city-states requests, I can get bonus from a majority of city-states. In civ VI, if envoys are hard to earn, attacking some city-states with abundant resources or good location is valuable, since you can not earn its bonus given the limited number of envoys.