https://wccftech.com/nintendo-switch-titles-cloud-saves/

The Nintendo Switch Online Service will finally launch this month worldwide, and with it will also come Cloud Saves. There has been some confusion regarding the feature in the past few hours, but it seems like all games will support Cloud Saves by default.

Speaking with Nintendo Life, an unspecified trusted source confirmed that all games will support the feature, but the developer has the option to opt out of it for a variety of reasons.