I have created a village-in-waiting for populating.  I have been attempting to populate by curing zombie villagers that should spawn each night.  I have been waiting for over 20 game nights, and have killed over 100 spawned zombies (close to 200), but no zombie villager has arrived at my location in this time.  It is as if zombie villagers no longer spawn in this world, though there were a couple early on in this world, in the first 20 game nights.