Originally announced last year, the Nintendo Switch Online service was to be Nintendo's first ever subscription based service for online multiplayer and online features. This was the initial announcement from 2017:
Users would have three time frames to pick from to subscribe including playing 20 dollars for a full year's service. There is also a mobile app available that allows voice chat and game invitations, but nobody uses that app outside accessing the Splatoon 2 gear shop. However the subscription service has been delayed once and once again has been delayed again.
This comes from the latest strategy briefing...
Next let's talk about Nintendo Switch Online, the paid subscription service that expands the online functionality of the Nintendo Switch. We reported previously that the service would launch in 2018, and now the official start time has been set to September 2018. Work is progressing on ways to further heighten the gaming experience for consumers. Details will be made available as they are finalized.