It doesn't have to be but it's good practice to remove it. These batteries can go up in flames on rare occasions, destroying everything nearby (very rare).
DJI Mindy Posted at 2018-4-24 22:38
It is recommended to remove the battery from the aircraft if you do not use it for a long time, please check more details of how to maintain the battery here: http://dl.djicdn.com/downloads/inspire_1_pro/en/Inspire_1_%20series_Intelligent_Flight_Battery_Safety_Guidelines_v1.2_en.pdf[view_image]