The /spreadplayers command is able to position entities outside the world border.

How to reproduce:
Run the following commands in order:

1. /worldborder center ~ ~
2. /worldborder set 10
3. /spreadplayers ~ ~ 0 100 false @s
   You now most likely ended up outside the world border.

For commands such as /teleport, you have to enter a specific position to be teleported to, so it seems intended that you can leave the world border using it. However, leaving the world border is probably not the intended behavior for /spreadplayers, considering that it's supposed to spread entities to safe locations.