It depends what sweetener is in it. You gotta look up what sweeteners are cool during a fast, then find a zero calorie drink that uses that sweetener.
There's one called Zevia that uses Stevia which I hear is cool, I wanna look more into it and try it one day:
The one I drink right now is called Sparkling Ice, which uses Sucralose. I'm about to stop drinking those because they have Maltodextrin which is supposedly worse for you than sugar.
But Sucralose is a big debate in the I.F. community.
Because some "zero calorie" drinks use Sucralose, which gives drinks a sweet taste without adding calories.
The debate is "does Sucralose (which are in these drinks) spike your insulin levels?" Some say yes, some say no.
One of the reasons I.F. works is because it keeps your insulin levels low and after fasting for at least 16 hours, your body starts burning your body fat.
If you have something that spikes your insulin level, it throws off the whole process and breaks your fast.
Me personally, I would not drink my Sparkling Ice during the fasting window just to be safe. I just have it with dinner because after drinking water all day, I just need something else.