"icing conditions" is a term often thrown around without a lot of definition. You can fly in cloud for years at -20C to +2C and never pick up ice. You can have the sponsons and wipers caked with ice and never see the torque change. Do we want to talk about potential icing conditions, forecast icing, known icing....? Give the manufacturers and regulators some rope on icing language and you'll guarantee you'll never fly.
Are you talking about RFM limitations or what actually happens on a day to day basis?
At -30 and -40 the air is dry and not usually much of an icing problem. Supercooled ground fog can be a problem as it can build up on the blades pretty quick. Not much flying goes on when it gets below -40. Old aircraft like the 212 and 61 fly IFR in the arctic and antarctic all the time. The 76 too, although it can't carry the ice a 212 can. By comparison, I've never seen any of the 139 guys poke into a cloud when the OAT gets less than +2. I hear they want to operate a 139 out of Tuk this winter so we'll finally see how that type carries ice. With all that power I expect it would do pretty well.
Airframe icing has never seemed to be a problem to worry about with any type. Its the stuff building up on the main rotor blades and driving the TQ up that gets your attention.