With the launch of the paid Nintendo Switch Online service came an additional update to its related voice chat app. What was previously a glorified Splatoon 2 helper app now includes support for more Nintendo games as well as a somewhat surprising new feature for a Nintendo console - voice chat with strangers.
The Nintendo Switch Online app now includes voice chat support for Splatoon 2, Mario Tennis Aces, Arms, Mario Kart 8 Deluxe, and the collection of NES games that come with the paid subscription service. For three of those games - Mario Tennis Aces, Arms, and Mario Kart 8 - voice chat is supported not only between friends, but also in lobbies such as Mario Tennis' free play and Mario Kart 8's global races that put you in lobbies with strangers.
NES Online does not allow voice chat with strangers as it does not include a mode where you can play the game with others who are not on your friends list.
Splatoon 2 also does not allow voice chat with strangers, though its exclusion as other games allow the feature is a bit more unusual as the game's primary play mode involves competing both against strangers and alongside them, including in ranked play.