Nintendo Switch will have a paid online service starting this fall - Polygon
Everything you need to know about Nintendo Switch Online | Digital Trends
You need a paid membership to Nintendo Switch Online to play course world in Super Mario Maker 2. Pretty sure you could access course world at no extra cost in the original
Nintendo Entertainment System™ - Nintendo Switch Online for Nintendo Switch - Nintendo Official Site
Nintendo Switch Online: a Complete Membership Guide
Nintendo Switch will have a paid online service starting this fall - Polygon
Everything We Know About Nintendo's Paid Online Subscription Service - IGN