Windows
C:\Users\%USERNAME%\Documents\My Games\Sid Meier's Civilization VI\Saves
Linux:
~/.local/share/aspyr-media/Sid Meier's Civilization VI/Saves
Steam Linux using Proton:
~/.steam/steam/steamapps/compatdata/289070/pfx/drive_c/users/steamuser/Documents/My Games/Sid Meier's Civilization VI/Saves
Steam using Flatpak:
/home/matmarspace/.var/app/com.valvesoftware.Steam/.local/share/aspyr-media/Sid Meier's Civilization VI/Saves