The bug
Ever since 1.9 when the end islands were introduced, a bug has existed where there are methods of going to the end islands without beating the ender dragon. Such methods include building a very long bridge there or making a slime block machine. I think this is a bug because the outer end islands are supposed to be post-dragon.

Possible fixes for this bug
One possible fix for this bug is to have the outer end islands in an entirely separate dimension from the ender dragon island, requiring the player to enter the end city gate to get here. Another possible fix is to have an impenetrable wall around the central end island, similar to the world boarder, once again requiring the player to enter the end city gate to get past this wall.