Here is a pair of numbers that looks like a mistake. Of the 217 sodas we have checked, 103 pass our keto rules, the best rate of any category. For vegan, only 12 of those same 217 get a clean yes, and 177 sit in our "check the label" pile. Same drinks, opposite outcomes.
Why keto loves soda
The keto question is arithmetic. A diet cola has zero grams of carbohydrate per 100ml, the label says so, and that settles it. No other supermarket category has so many products that are literally carb-free. Compare cereal, where 5 products out of 403 make the cut.
Why vegan cannot trust it
The vegan question is about origins, and soda labels are built to hide them. The core of almost every soft drink is "natural flavourings", a phrase that legally covers extracts from plants and, in rare cases, from animal sources. The formula is a trade secret, so nobody outside the company can verify it. Some colourings raise the same question: E120, the red pigment in some fruit drinks, is made from insects.
Open Food Facts marks most sodas "maybe vegan" for exactly this reason, and we pass that on honestly. It is not that your cola probably contains animal products. It almost certainly does not. It is that nobody can show you the evidence, and "probably" is not what a checker should sell you as "yes".
The takeaway
A question that can be answered from the nutrition table gets you a firm answer. A question that depends on undisclosed recipes gets you an honest maybe. If a brand matters to you, their customer service will usually confirm vegan status in writing, and that beats any database, including ours.