This issue has been raised in the thread on Longo’s fast-mimicking -diet. It’s a 5-days very low-calories, low protein, vegan stint. Conceptually, it may be harmful. Pra tically it may be not. There are no studies on it apparently.
Edit: While re-reading my question, it appears that a low-carb version of the FMD might avoid to overstimulate insulin secretion, especially if simple carbs/sudden peaks are avoided.
In Water only, short fasts, there would be no glucose input hence no stimulus to insulin secretion hence no amplification of a glucose signal which is not there.