I got the warning when my battery was slightly under 15C (yes, C, not F). It's not in the firmware, it's in the Go App. I still have the 1.5 firmware.
I still wonder what's the minimum temperature you can fly full speed ahead with a cold battery. I keep an eye on the voltage, particularly when I flew with the warning on, and I know that the cut off where the battery shuts off is at 3.00V. The recommended minimum by DJI is 3.20V (saw it in an e-mail from DJI). I always try to keep it above 3.40V. I wonder if I can fly with a cold battery (say 15C) and as long as I'm careful not to let the voltage drop below say 3.40-3.50V, that I should be fine. Seems common sense, but I really don't know how LiPos behave in cold climates. Could it possibly go from 3.40 down to 3.00 in an instant (under some failure condition)?