Nintendo is almost always reliable when it comes to indicating on cases what you need to fully enjoy the games inside them. Aside from the legalese you can find what controllers are compatible, the number of players the game supports, and now whether or not you’ll need a Nintendo Online Service (NOS) account to enjoy online play.
Twitter user @ibbsters provided visual confirmation of Nintendo’s NOS notice on the back of a Switch game case. In this instance, the case reveals a Nintendo Switch Online account will be needed to enjoy the game’s multiplayer aspects. There’s no indication if a notice will be present if the service isn’t required, but more than likely such information will be omitted.
— sir digby chicken (@ibbsters) September 18, 2018
No doubt future multiplayer 3DS games will have their own notices, so get used to flipping over game cases if you plan on playing online.