Firstly, slavery should be over when the player (or their civ) says so, and not before. However, that's an argument for another time, because I already said this is post-slavery. Secondly, feudalism is unlikely to be completely over. There are half a dozen or more feudal civics so it's likely that almost everyone is still running at least one. Even conceding that diplomacy should be what they're about, what's your ETA on that? Currently they have no diplo function whatsoever. Furthermore, many of them are barbs/rebels (enemy combatants/insurgents lol), with whom diplomacy does not apply.