Not really. While there are some sugar subs without calories or carbs, they still make the digestive system do work. So if you’re fasting to give your digestive system a break, I’d say its best to avoid altogether.
Water + black coffee are your friend.
I eat 11am to 7pm (16:8). But, if it weren’t for my morning (11am) coffee with cream/sugar, I could easily switch to 18:6 or maybe even 20:4, because I don’t eat a proper meal until mid-afternoon. I wish the coffee didn’t count for breaking the fast. Alas!
Online protein shops like Myprotein sell sugar free sweeteners which I find pretty good. I add a couple drops for my black coffee occasionally (only after breaking a fast though so as not spike my insulin while fasting ).