Message content produced: zero, and not as a policy
We do not decline to hand over message content. We cannot. What this service holds is ciphertext and wrapped keys, and it can read neither. A demand for the contents of an account's messages can be complied with completely and fully, and what it yields is material nobody involved can turn into words.
That is why this figure is printed exactly and never banded. If it is ever anything other than zero, a range would be the wrong way to tell you.
The limits of that claim are on the delivery page and are not buried: encryption in a browser protects you against a server that stores your messages, and not against one that serves you modified JavaScript. We say so because it is true, not because anybody asked.