Mobs that touch the world border start to take damage from suffocation.
Mobs can walk into the border by themselves and cause harm to themselves, but for the video demonstration I simply push the villager (which can by any mob) into the border myself.
Video: https://youtu.be/pQhcCnQn1iA
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Testing this in 1.16-pre7 it doesn't seem like mobs suffocate in the border anymore.
I made a short demonstration of this: https://youtu.be/bJNhwfV5fSs
Can this be reproduced in 1.16-pre7?