https://www.gamespot.com/articles/fortnite-on-nintendo-switch-doesnt-require-an-onli/1100-6461835/

Nintendo is rolling out its paid online service for Switch very soon, and once it's live, you'll need to have a subscription in order to continue playing most Switch games, such as Splatoon 2 and the upcoming Super Smash Bros. Ultimate, online with others. However, a handful of games will be exempt from this requirement, including Epic's massively popular battle royale game, Fortnite.

Just as on PS4 and Xbox One, you won't be required to have an online membership to play Fortnite online on Switch, even after the system's paid service launches. As Nintendo clarified in an FAQ on its support website, Fortnite is one of the few online multiplayer titles that can be played without a Nintendo Switch Online subscription.