https://www.gamespot.com/articles/apparently-nintendo-doesnt-just-erase-your-cloud-s/1100-6462044/

One of the more notable features included in Switch Online is cloud storage, which allows you to both access and recover your game's save data even if your Switch is ever lost, stolen, or damaged. A Nintendo FAQ page created some concern that Switch Online would immediately delete your cloud storage if you ever stopped paying for the subscription, whether on purpose or not. Nintendo has now clarified that the FAQ statement is only partially true.

Switch Online will delete your cloud saves if Nintendo doesn't receive a subscription payment, but the action won't be taken immediately. According to a response made to IGN, cloud saves can be recovered up to six months after your subscription ends. You won't be able to access your saves once your subscription expires, but they'll remain on Nintendo's servers for 180 days before they're gone for good.