Since 19w45a?, hostile mobs which are located in lazy and non-ticking chunks do despawn, and they didn't previously, and that was useful for things like mob switches based on a mob spawner.

Note: it's expected for mobs to despawn if you would keep them fully loaded while walking away for more than 128 blocks, but previously, if you'd leave an area by a portal and keep hostile mobs in lazy chunks, they would stick around even if you'd be somewhere else in the same dimension. Not anymore.