can some kind hearted soul please tell me how i can restrict access to "The Abyss" untill the ender dragon has been slain?

im a complete noob at coding sorry

You would create a new stage called "abyss" or something similar, and then stage their dimension to that stage. You can then use the StageHelper to give a stage when the advancement is given to the player, or you can specifically track when a mob dies.