I suppose one could route an exhaust stack from the engine installation to a location away from the engine or propellers. But then again the benefits would have to outweigh the drawbacks of such a design.
Adding additional exhaust routing would solve the problems of structural damage due to heating, But it’s also bulkier, heavier and imposes additional design requirements for heat shielding the aircraft structure, preventing exhaust gas leaks and awesome designing an exhaust outlet installation to vent the exhaust gases away from the airframe somewhere else on the aircraft. And that’s not really necessary if the temperature from the exhaust gases around the propeller blades is sufficiently cool enough to not cause structural damage the blades. A PT6 usually has a maximum Inter-turbine temperature (ITT) around 750° C but the temperature of the exhaust gases exiting the exhaust stubs Is going to be lower-probably only order of 400°C or so. Add intermixing with outside air flow and the temperature of the exhaust gases may be down as low as 250 to 300°C by the time it passes through the propeller arc. This is going to be far from hot enough to damage the propeller blades provided their manufactured out of the right material to the right specifications. It has an additional bonus and that the hot exhaust gases can serve as a means to prevent ice accumulation on the propeller blades during flight, thereby forgoing a requirement for hot props or another kind of prop anti ice solution