https://www.polygon.com/nintendo/2017/9/23/16355314/nintendo-network-two-factor-authentication

Nintendo Network, the portal for users on the Nintendo Switch and 3DS, implemented two-factor authentication for all accounts effective now.

The feature is available in the Nintendo Network portal after logging in; the user then should go to the “sign-in and security settings” tab. At the bottom of that column is an option to enable two-step verification.