A headline seen in Elliot Mills's office reading "Final Frontier?" might refer to the Frontiersman's death. Given that he smoked it would make sense that he might have gotten lung cancer and fell victim to that.
Again it's possible that he wasn't an actual hero but a character made up for marketing purposes. But on the other side it's actually possible that he was a true-blue hero who did advertise for Frontier Cigarettes. It would be similar to many baseball players of the 1940s and 50's who did such advertisements.