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Animal Crossing: New Horizons will have a save backup function for Switch Online users
Not that long ago, people noticed that the eShop listing for Animal Crossing: New Horizons didn't include mention of the Switch Online cloud save backup feature. It does appear that that feature won't be supported, but Nintendo is going to remedy the situation another way.
Nintendo has shared a feature on their Japanese site about Animal Crossing: New Horizons that says they're planning a save backup function for Switch Online users. The way things are worded make it sound like this will be an option outside of the usual cloud save backup that Switch Online users normally use. We'll have to hold out for more details on what this backup is and how it works.
UPDATE - We have a more specific translation of the notice Nintendo shared, which roughly says the same thing. Thanks to AutumnalBlake for the heads up!
This game does not support Nintendo Switch Online Cloud Saves, but we are currently looking into a customized method for backing save data up that will be specific to this game in the event that your Nintendo Switch is damaged or lost. This function will be limited to Nintendo Switch Online subscribers. The timing of implementation is undecided and will be announced as soon as it is finalized.