By using the /summon command, you can now summon entities that can't be forced to despawn whatsoever.

Try for exemple : /summon Bat ~ ~ ~

Bats don't despawn when difficulty is set to peaceful, the "PersistenceRequired" tag prevents them to despawn when far away from players and the "Invulnerable" tag prevents them to take any damage.
(Note : I'm not sure about void damage but it doesn't matter since Bats can fly and thus can't be forced to fall into the void)