MyNintendo is a rewards program that Nintendo offers to Nintendo Account holders, where any purchase made digitally on the eShop gets you 10% back in credit in the form of gold coins; this credit can then be used towards purchase of further games on the eShop. Apparently, it can also be used to purchase a Nintendo Switch Online subscription, as it turns out.
Nintendo revealed this on the Nintendo Switch Online FAQ page of their website, also confirming that the coins can’t be used for auto-renewals. Since they have to be manually redeemed for eShop credit, that much at least makes sense (though you can always just manually purchase a new subscription using those coins, if it comes to it). If you purchase a lot of games digitally, then those coins can help bring the cost of the subscription down substantially. If you’ve been saving them up, this might be a good time to use them.
Nintendo Switch Online launches tomorrow.