Can not go above the world hight of 320
but
there are some places where you can go above

By the way, i am holding space bar

you can go on top of the border, if you wear an elytra, in creative and hold space bar, then quickly double tap to fly, then you glitch though the invisible border

Here's a small clip of this bug
https://youtu.be/Wk1iogH6KUw