It will be for some games. What the FAQ says is:
“Nintendo Switch games such Splatoon™ 2, ARMS™, Mario Kart™ 8 Deluxe, Mario Tennis™ Aces, and Sushi Striker™: The Way of Sushido, will require a Nintendo Switch Online membership, but other games will vary.“
So most likely first party games will require it, and some third party, but games with smaller online features would have online available.