Villagers from old worlds, do not accept new job blocks and have the AI of an unemployed villager (shaking their head, sleeping late) while still being able to trade, and have a trading UI. This also gives them infinite trade with no need for refreshing them. Villagers like farmers also do not farm.
This can be taken advantage of and exploited.
I assigned each of my librarian villagers from 1.13.2 a lectern without a problem. You can use /data get entity <villager UUID> Brain to debug the villagers (you need to be op though).