Epic Games and Nintendo will not require you to pay the Nintendo Switch Online subscription to play Fortnite.
As reported by Nintendo itself through an official statement on its website, the subscription of Nintendo Switch Online will not be required to play some multiplayer titles, giving as an example Fortnite. “Membership in Nintendo Switch Online will be required to enjoy the cooperative and online features on many Switch games such as Splatoon 2, Mario Kart 8 Deluxe, ARMS and Super Smash Bros. Ultimate.
Online play will be free until the Nintendo Switch Online membership launches on Sept. 18, 2018.