I don't think any dev policy would be based on a strict year cutoff, like 1945 being okay but 1948 not being okay. Rather it's about not upsetting people, so a good rule for that is, don't include leaders if there are people still alive with strong negative feelings about the leader. That's why John Curtin, who died in 1945, is acceptable - as I understand it, he's viewed as a positive figure in Australia and is certainly obscure elsewhere - while for instance Vladimir Lenin (died in 1924) is impossible. There's nobody alive who remembers him but he's the founder and main symbol of a country that many people have strong feelings about so putting Lenin in wouldn't be acceptable.
This is not to say that one can't imagine a Civ-like game with controversial and outright hated 20th century leaders in it, but that's not going to happen in an official Civ game now. 2K of course has veto powers over any externally visible part of the game, and 2K has, beyond any doubt, shown itself to be a very cautious company in terms of communication, marketing and appearances. It's clear that 2K would not sign off on anything that could be anticipated to create controversy.
