It looks like some games may be exempt from paying the Nintendo Switch Online membership fee to participate in online play.
On Nintendo’s support pages, it appears not all games will need players to pay the yearly $20 fee. The only game mentioned specifically was the popular battle royale title Fortnite, though it seems other games could be exempt as well. Nintendo also states that titles that do require the membership for online play will be identified in the Nintendo eShop beginning September 18.
The page states: "Beginning September 18, 2018, a Nintendo Switch Online membership is required to participate in co-op and competitive online features for many first- and third-party Nintendo Switch games, including Splatoon 2, Mario Kart 8 Deluxe, ARMS, and Super Smash Bros. Ultimate. Some games, such as Fortnite, can be played online without a Nintendo Switch membership."