The following comes from a source, who spoke with Nintendo Life about Cloud Saves on Nintendo Switch...
All existing games will default to “enabled”. I specifically asked Nintendo about that and had that response. If a dev/pub wants an existing game to not support it, they have to ask.
When submitting a game, you are encouraged to enable cloud save. You have to have a good reason to not support it.
Reasons to not support it are generally based around preventing a user abusing it to cheat in your game. I can’t think of anything beyond Animal Crossing being too concerned about players rewinding their saves, but some games that give a random weapon drop, for example, might want to also prevent rewinding.
This system is something we've heard before ourselves, which adds credit to the idea of the opt out request. Which games do you think won't support it?