Yeah helmets are absolutely mandatory for all downhill sports regardless of if you're snowboarding or skiing. The last 10 years I haven't seen anybody in any of the mountains here without helmets (something changed in this regard about 15 years ago I think when you could see people without helmets still).

But the next most important protection for snowboarding is a wrist rail protecting your os scaphoideus, it's the larger bone in your wrist (just at the base of the thumb) and it's the most common snowboarding injury as you fall backwards and instinctively reach out with the hands behind you and the impact most often fractures the wrist bones. This happens even with you standing still!