Black coffee and herbal teas don’t negate the positive benefits of fasting, however, there is a lot of evidence to show that sweeteners can ruin a fast. There are also people who provide evidence that says it doesn’t. If you’re fasting for weight loss only, then some sweeteners may not be a huge deal (although I’ve seen people say that it does slow their weight loss). If you’re fasting for autophagy and other health related reasons, then I would avoid them completely. I’m not expert, though.
I drink coffee and energy drinks with added artificial sweeteners. I’m still productive and I still burn calories for just staying alive. I think the weightloss from fasting is more important to me than other benefits that MAY be canceled out..