https://www.gamespot.com/articles/apparently-switch-online-will-delete-your-cloud-sa/1100-6461803/

Although the much-requested cloud saves system is coming to Switch, there's apparently a pretty big catch. If you ever stop paying for an online subscription for your Switch, Nintendo won't hold on to your save data and it will be deleted.

According to an FAQ page on Nintendo's website, "Save data stored with Save Data Cloud cannot be kept outside of the duration of your Nintendo Switch Online membership." Nintendo then adds that if you keep your save data stored "locally on your Nintendo Switch console, then you can use it again if you purchase another membership."