Should be fine. No sugar, no real calories, and no artificial sweeteners either (which the most stringent fasters recommend avoiding).
You may have a couple iffy responses here because some people get very strict about it, but it’s not worth making yourself that crazy. If a little fruitiness helps you push past the hard parts, it’s 100% worth it.
I personally do a matcha green tea (0 calories, but a richer flavor than plain green tea) in the mornings and a lavender chamomile tea in the evenings.
Cost vs value, the goal is to reduce insulin the longer the better everyone reacts different to little drinks of this and that, if you need a little post go for it if you find ur not winning at goals then reduce them
Especially if one drink helps you complete for example your 16 hours fast etc
However cutting it all out while hard at start makes it all easier long term
I would say if it can help you get through this slightly more comfortably it is worth it. Look, a fast isn’t black and white. There are so many routes you can take and you just have to find the best one for you. Best of luck!
There seems to be a wide range of what constitutes breaking a fast. I heard recently that anything under 50 calories won’t harm a fast and on the other end the likes of Rhonda Patrick says breaking a fast includes anything that’s not water. I’d say fruit flavored teas are on the safe side.