Disclaimer: Before this is marked as 'Works As Intended', that is incorrect. Zombie Villagers have always had a chance of spawning from a Zombie Spawner. I'm making a new issue, because if I comment on the WAI issue, it'll simply get ignored, and the issue will remain.

Zombie Villagers have a 5%, or 1 out of 20 chance of spawning naturally, or spawn from a Zombie Spawner. In version 16w44a, no Zombie Villagers, and its baby counterpart are not spawning from Zombie Spawners.