Worldborder does not damage mobs
It seems like the worldborder damage is not working in the latest snapshot
- Set worldborder center
/worldborder center ~ ~
- Set worldborder size
/wordborder set 10
Watch mobs outside the worldborder, they won't get hurt, if there are no mobs you can teleport outside
/tp ~ ~ ~13
and then summon some
/summon Pig