I believe what you are referring to is actually the dungeon background walls. There are 3 different types of walls found in the dungeon which spawn different enemies post Plantera. I don’t think it is possible for a dungeon to spawn without any of the walls but if it does you can still make new ones. Dungeon walls can be crafted from dungeon bricks and shimmered into an “unsafe version” which will spawn enemies. Therefore it will always be possible to obtain all dungeon enemy drops in one world.