If you decide to go with a Switch Online subscription for an amount of time and then cancel later on, say goodbye to your cloud saves. Nintendo's Switch Online FAQ states that if you let your subscription lapse or you cancel it, your cloud saves will go up in smoke.
Save data stored with Save Data Cloud cannot be kept outside of the duration of your Nintendo Switch Online membership. Nintendo Entertainment System – Nintendo Switch Online also uses the Save Data Cloud, so the same applies. However, if you keep the Nintendo Entertainment System – Nintendo Switch Online save data saved locally on your Nintendo Switch console, then you can use it again if you purchase another membership.
It's unclear how quickly your cloud saves will be deleted, but it doesn't seem like they'll stick around for too long.
We are unable to guarantee that cloud save data will be retained after an extended period of time from when your membership is ended. However, you can continue to use the save data that is saved to your system memory.