https://www.theverge.com/2018/9/18/17874798/nintendo-switch-online-launch-subscription

Nintendo Switch Online is scheduled to launch later today, September 18th, shortly after 11PM ET / 8PM PT. Once it arrives, the free online gaming that Switch owners have enjoyed since the console’s launch will come to an end. To enjoy the same games online, you’ll need a subscription to the service.

Thankfully, buying into the service doesn’t cost very much, and it will grant you access to more than just online gameplay. You’ll also be able to chat with friends via the Switch Online app, back up your precious game saves to the cloud (though there are a few caveats), and play a growing collection of retro NES games online or with a pal locally. Those games require an active Nintendo Switch Online subscription if you want to keep playing; they’ll be inaccessible if your membership ends.