The Nintendo Switch version of Civilization 6 does not have online multiplayer, publisher 2K has confirmed to Eurogamer.
Multiplayer is limited to local play only.
Alarm bells began to ring when perceptive Reddit user WinneonSword spotted something a little odd with the Civilization 6 Switch eShop listing, which specifically states one to four people can play over “local wireless”. Similarly, I checked out the British version of the store, which says “up to four players can cooperate or compete for supremacy via a wireless LAN”. Unlike other upcoming multiplayer games on the store (such as Warframe, Diablo and Pokémon Let’s Go), there is no mention of online multiplayer in the Civilization 6 eShop description.
Source: Eurogamer Civilization 6 on Switch does not have online multiplayer