Because of the 1.14 villager changes, villagers can change their employment based on their work block being destroyed. I think an event to detect this change (and cancel it if wanted) could benifit developers and servers who want to make plugins to change how villagers change employment or disallow villagers from changing professions.

Though a new event could make sense, this could be integrated into EntityTransformEvent but having the ability to track what a village's previous profession was wouldn't work too well in Entity Transform because it would only effect villagers. 

Edit (17/05/19 9:15pm CT): I have decided to implement this feature into code base and am currently awaiting CLA approval. I have figured out what code is triggered when a villager's profession changes. I am currently writing the event and implementing it.