While I don't have experience giving plants carbonated sugary beverages, the general opinion people I've encountered online have is that the sugar would be bad for the plants, might invite mold and other microbes, etc. I'm guessing the rest of the ingredients wouldn't be terribly toxic to plants, though.
Questions like this have been asked on gardening sites plenty of times, but usually they're wondering about the phosphoric acid in Coke (which doesn't exist in all soft drinks). Phosphoric acid can be good for plants, but you probably don't want all that sugar with it.
The sugar would probably be converted to alcohol and then to vinegar (acetic acid), eventually, is my guess. Acetate minerals can be good be good for plants, but concentrated vinegar when the plant is exposed to sun can kill plants.
According to this site, sugar can kill plants by causing reverse osmosis to occur. I don't know why reverse osmosis would kill plants, but I haven't studied it in that regard.