According to the book, it's a no no to anything other than black tea, black coffee & water. I don't think Malic Acid is believed to have the same effect on ketosis, but it's best to avoid any soft drinks if you possibly can as they can stimulate your appetite.
I have *never* indulged in a Coke zero, perfectly clear strawberry & kiwi or Asda no added sugar cream soda, which don't contain citric acid, honest guv.