Pillagers are the only mobs (apart from phantoms, see MC-127599) that continue to spawn in the void biome. When I create a superflat world using the void biome no other mob except pillagers (and phantoms) continue to spawn naturally.

Steps to Reproduce:

1. Create a superflat world with the following preset:

   minecraft:bedrock,2*minecraft:dirt,minecraft:grass_block;minecraft:the_void
   

2. Change the world difficulty to Hard
3. (optional) place a repeating command block (set to either always active, or active with redstone) with the following command:

   tp @e[type=!player] @p
   

4. Tick sprint the game by a large number of days
5. Wait for a patrol to spawn (can leave the game running in the background. You will know when a patrol as spawned because they will make noise once teleported to you)