https://www.gamespot.com/articles/nintendo-switch-online-nes-games-pricing-family-pl/1100-6461212/

Nintendo fans have had it good for a long, long time. Unlike Xbox and PlayStation owners, they've never had to pay to play their games online. This free period, like all good things, has come to an end. With the launch of Nintendo Switch Online--which is now live with the 6.0 system update--Nintendo has begun charging for online play, although there is a free seven-day trial available to sample it first.

As with Xbox Live Gold and PlayStation Plus, anyone who wants to get the most out of their Switch will all but have to subscribe. (Note: this doesn't affect Wii U or Nintendo 3DS owners; online service for those systems will remain free). And while it's not ideal having to pay for something you used to get for nothing, you won't have much of a choice.