Eh, this is one of those purist diatribes.
Straight coffee or tea has no calories and not much of an insulin response. So if you are doing intermittent fasting for weight lost (most people) then it's fine. See https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5583402/ for a meta analysis of 7 studies done on green tea.
If you are trying to go for apoptosis and other kinds of metabolic effects, intermittent fasting likely isn't enough to get you there to a significant degree anyways. You need to be doing 2-3 days fasts which falls under the "extended fasting" category.