World Border

The 9b9t World Border is present in all three dimensions; the Overworld, the End and the Nether.

Overworld
The world border in the Overworld is the easiest to reach due to the convenience of Nether travel. Travelling one block in the Nether is equivalent to travelling 8 blocks in the Overworld. This makes Nether travel far faster than Overworld travel, hence its popularity.

There are numerous bases at the world border, including Jeff Island, and Rigabase II

The End
The End border is far more difficult to reach. Due to the fact that the End is largely made up of islands, one can catch themselves on land if they are about to fall into the void.

Nether
The Nether border is much more difficult to reach due to the fact that there are no travel shortcuts.

A 9b9t player by the name of Inzoide has reached the Nether border. The video can be seen here: https://www.youtube.com/watch?v=_OWOwfy6WlM