In episode 412, “Darkness on the Edge of Town,” Cruella held a gun to Rumple’s head and told him that outside the town of SB, Rumple could die a mortal death. Rumple agreed. Now it’s very possible and even probable that Rumple was lying to Cruella and it does feel like a retcon from what we know about the Dark One’s curse, but let’s face it–this show ain’t exactly consistent.
I think you could definitely be right about the writers having Rumple die by season’s end, although I hope he doesn’t – we all know the show would suffer without him. But I’m confused by you saying his being able to be killed outside of Storybrooke in our world as unusual. I agree this show breaks its own rules, but haven’t they been consistent on this?