I'm not going to say that a seatbelt is an absolute must-have when you're launching your old Ford Ranger many feet into the air, because I've never jumped a truck before. But looking at this video evidence, I think it might be a good idea.
As you will not in the following video
the guy's head clears the roof line of the truck entirely. That's probably not good.
But I shouldn't worry - he's wearing a helmet. I'm sure that means he's safe enough.
(Hat tip to Sweet Jumps, bro!)