Disney reserves the right to change menus and dining plan offerings whenever they wish to, so there is no telling whether they will leave the lobster on the dining plan or take it off, or when they might do so. They are far more likely to remove something from the menu altogether rather than exclude it from the dining plan. That has happened on a few occasions. Most of the time if they want to cut back, they just continue to offer the dish, but use lesser quality ingredients.
As of now, you can still order lobster at Narcoosees for two TS credits.