CreatureSpawnEvent's used to have the SpawnReason as "CHUNK_GEN" if the creature being spawned was due to first-time chunk generation. Now, no creatures fire the event with "CHUNK_GEN" at all.

Recreating:

1) Fresh Spigot/CraftBukkit install with no other plugins.

2) Create a basic plugin with the following event registered to the PluginManager:

@EventHandler(priority = EventPriority.MONITOR)
public void onCreatureSpawnEvent(CreatureSpawnEvent event) { 

   Bukkit.broadcastMessage(event.getSpawnReason().name());
} 

3) Confirm that you are receiving spawn reasons such as "NATURAL" in the chat.

4) Observe that as you load new chunks in a new world, the spawn reason "CHUNK_GEN" is missing.