https://www.pcgamer.com/civilization-6-art-director-releases-a-mod-that-makes-it-look-like-civilization-5/

Have you ever played Civilization 6 and thought to yourself, this is great, but I really wish it looked a little... older? If the answer to that question is yes for some reason, Civ 6 art director Brian Busatti has come up with just the thing: A Civ 5 "Environment Skin" mod that's now available through the Steam Workshop. 

The mod "changes the visuals of the game to better match the colors and tones" of Civ 5, with changes to texture details, a more neutral color palette for buildings and units, and other changes.