Given the history of the series and some of the creations people have made, it was always a little weird that Civilization 6 shipped without official mod tools and Steam Workshop support. But for those holding on hope, good news. They're coming. Eventually.
In a stream through the official Firaxis Twitch account, Civilization producer Pete Murray confirmed that Firaxis was working on Steam Workshop support, as well as official mod tools and the ability to have multiplayer teams.
There are plenty of mods for Civ 6 right now, but they're in the same sort of status as mods for Fallout 4 before the official tools were released. They're written without the support and grounding of the game's SDK, which makes it harder for modders to unleash their ambition. It also means existing mods have the potential to break whenever official tools are released, and there's always the prospect of new patches screwing everything up. The ease of Steam Workshop isn't to be overlooked either: anything downloaded through their auto-updates, and discovering new mods is a pretty simple process.
Firaxis declined to give any details on what or when anything might come out, but as soon as we know more we'll let you know.