You can keep losing weight any time as long as you're in a calorie deficit. It's easiest to lose weight early on because (1) your restriction is strongest, so eating at a deficit is practically automatic, and (2) you have the most excess weight, and the more excess weight you have, the easier it is to lose.
As you lose weight and become smaller, the calories you burn just by existing (breathing, circulating blood, digesting food, moving around) get lower. If you weigh 200 pounds and eat 1200 calories per day, you'll lose weight a lot faster than if you weigh 150 pounds and eat 1200 calories per day. You can still lose weight, but you may have to be a lot more conscientious about your food intake instead of just relying on your surgery to prevent you from overeating.