Officially, we know a little from the Monstrous Manual and other sources like Volo's. I think their intention is still to describe what they (Yuan-ti in this case) are, but not in the race description. That information must come from elsewhere, possibly from setting-dependent sources.
So far D&D has been operating under a "this is how things are, except for when it's different" philosophy. It feels to me that they want to start operating like "It always depends. In thins setting, things are like this" instead.
So I don't think WotC is trying to remove moral/cultural information from the game, but to migrate this information to another place than PC racial templates. Which other place? I don't know yet. I hope they do.