Slavery isn't absent because of increased 'political correctness', whatever that means. I remember the advertising for IV, 13 years ago, and how they went out of their way to demonstrate that they would deal with slavery and religion carefully, that slavery brought negatives to civilisations and that religions would be treated equally, etc. The fact that they've changed the religious system so much that you can create religions named, say, Catholicism or Islam that use their symbolism but have features supposed to represent theology and doctrine in many cases that have absolutely nothing to do with those religions, and in many cases are borrowed from paganism or heresies, shows how this is just a complete non-issue. Only a handful of nutters would care.
Slavery is more of an issue for the obvious reason that there are millions of people globally, particularly in the countries where people play these games, with pretty resent descendants of an extensive system of slavery with a fundamentally racial basis, which has a legacy of systemic racism and inequality. But again, no-one cares that Europa Universalis IV has slaves as one of the main trade resources of West Africa, and that you can increase the productivity of that resource and build trade notes to export them, because its accurate. If Paradox brought out a DLC with a feature that elaborated the system more fully, it wouldn't be an issue so long as it was well-researched and advertised properly. A 'slave DLC' would obviously be tasteless, but changing the way certain resources operate, and reflecting on the unique and human nature of the slave resource, would be completely doable.
There are important reasons why V and VI should have slavery mechanics, if we approach it from the point of view that Civ games should have crucial aspects of human history included: slavery and serfdom, in all of the eras of the game, are incredibly important in all their different forms, especially in periods of economic and societal change. However, if this is the case, then the game should have much more complicated revolution mechanics; it should have civil war mechanics; it should have world war mechanics; it should have a colonisation mechanic; it should have a vassalage mechanic; it should have religious heads; it should certainly have migration; it should have ethnicity, language and dialect mechanics, etc etc. But when they make these games they make a very deliberate choice to prioritise mechanics over historical accuracy (unlike Paradox, who very gradually try to work out how to create mechanics for everything), and so some of these mechanics have never been tried and some have been dropped.
Slavery is absent from this iteration (so far) simply because they couldn't figure out a good way of doing it, and to be honest I think that's an indictment of the government system, which has been going backwards ever since SMAC/CivIV.
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Get hyped! The day after tomorrow ... First Look video: developers show 'modders first gate to paradise', the new Lua API ... complete list of Civ6 Lua Objects in detail described, ready for download ... 
