You lose date & time, but nothing else seems to be lost. Testament to this are the numerous reports of bodies that require re-entering of date/date when main battery is removed, of which I had two. The K-01 and the Q did this. But there are plenty of other reports.
I don't know the technical terms but I assumed that camera settings would be written to an on-board equivalent of the SD-card type of memory, and therefore not need an internal battery to retain that data. Date and time would be a different matter, as that would need an internal electronic clock to keep running.