Spawners (Dungeons, Strongholds, etc...) can spawn mobs outside of worldborder
I put 9 pig spawners around grass, changed the worldborder so all the spawners were outside the border then watched as the pigs spawned, then died... repeat.
I put 9 pig spawners around grass, changed the worldborder so all the spawners were outside the border then watched as the pigs spawned, then died... repeat.