From the members I have conversed with, they are forbidden from using caffeine.
> It took a great deal of time, repetition, patience; no small amount of hope and faith; lots of reassurance from my wife; and many liters of a diet soda that shall remain nameless.
[0] https://www.churchofjesuschrist.org/study/general-conference...
This is the type of black and white response that I find so common with ex-mormons. If somebody pushes back on disinformation (even easily disproved like the above thing about keeping caffeine teachings secret), the superstitious thinking kicks in and excuses fly (like "they must be a secret apologist" which I heard recently). It's every bit as ridiculous as the believers are when they dismiss inconvenient facts like Zelph the White Lamanite[1][2] because it goes against their preferred narrative. It's superstitious thinking.
Edit: Hah! I couldn't have asked for a better real-time example to demonstrate my point: https://news.ycombinator.com/item?id=35774479