It looks like that you won’t need to pay extra just to play Fortnite on the Nintendo Switch.
Nintendo are launching a paid online service similar to PlayStation Plus or Xbox Live for its Nintendo Switch gamers on September 18th, and with it have released a load of information in an FAQ regarding the terms of the online service.
Nintendo announced that many first and third-party games on the platform, such as Splatoon 2, Super Smash Bros. Ultimate and ARMS, will require Nintendo Online to access online play.
However, it seems that some titles that cannot be played without an online service will not be required to limit their players to monthly subscribers of Nintendo Online. This category of titles of course includes Fortnite, which has no single-player/offline portion of the game.
This ruling is similar to the stance of Sony, who do not require players to have a PlayStation Plus subscription to play Fortnite Battle Royale with their friends.
Nintendo Online launches September 18th, 2018 for Nintendo Switch and will allow “users to participate in online gameplay, as well as provide additional benefits like a collection of classic NES titles, and will gift members with some exclusive, special offers, as well.” Those special offers are exclusive to the U.S. and Canada, for some reason.