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If you delete your Nintendo Account, then the Mario Kart Tour data linked to it will become inaccessible.
If you want to access Mario Kart Tour again, delete the app and reinstall it.
Note: If 30 or fewer days have passed since you deleted the Nintendo Account, it is possible to restore the account. After you restore the account and redownload the app, select the account your data was linked to on the Nintendo Account screen within the game to access your Mario Kart Tour data.