Not everything will require a Nintendo subscription service to play online: including Arena of Valor
Free-to-play exceptions reign, like other platforms Platform holders have a history of excluding certain games, including free-to-play titles (which may or may not have optional subscriptions or microtransactions), from the scope of their subscription-based services. In other words you can download a free-to-play game and still log in and play without Xbox Live or PSN Plus. There was some contention initially from companies like Microsoft but eventually publishers smoothed things over....