AFAIK, Updates are always optional, though DJI Go will keep pestering you with an update notification. But if you aren't bothered by that, you should be free to go as you please. My Air has been running on outdated firmware for quite a while now since it flies perfectly at the moment. If it ain't broke, don't fix it is my personal philosophy.
That's DJI's intention - for you to misunderstand it to mean that. They want unwary people to think that it is a required update (in order to fly). It's a sneaky way to get reticient people to update their FW. However, to date, all FW updates on the MP and MPP have been optional.