Civ 6 Capture City State
If you capture a city state in Civilization 6 it will give you 2 options:
Capture or Raze
You can raze city states forever. If you choose to capture them they will not be occupied and will behave as a normal city.
Liberation is a third option if you are recapturing a city state from another player.
NOTE: City state special bonuses are VERY powerful in Civ-6 and these disappear when you take over or raze a city state (in case of razing that special bonus is gone forever).