New Product - Maybe or Maybe not?
 

Heat shield for the diaphragm in the heater valve.
https://cimg7.ibsrv.net/gimg/rennlis...62ebb8071c.jpg
https://cimg8.ibsrv.net/gimg/rennlis...9aae2570e8.jpg
https://cimg9.ibsrv.net/gimg/rennlis...4b7ac60b54.jpg

.... and the heater valve failures I've had over the years were due to "warped" disk in the other part of the valve, not the diaphragm. YMMV

I thought the new Porsche ones came with a heat shield of some sort.

With it being so high up in the engine bay I don't think it would get as hot as other components that could use a heatshield like the PS lines.

This cover is only available for the heater valve fitted to all 928's from 78 to 95. Part number 986 307 507 00. Always been shown in PET I just never noticed it before.
On the basis that Porsche never designed a part without good reason I may add one to my GTS for grins.
I do not remember a heater valve failure for the diaphragm but always the disc valve.
I only sell the "Made in Germany" Mechano-Bundy valve which lasts way longer than all the others made in PRChina.

Metal retains heat for a longer period than plastic with a higher ranger and lower dissipation, so you will be making the problem worse from my humble opinion.

Heat shields are for blocking directional radiant heat, say from a really hot component nearby...like a cat, exhaust manifold, or the down pipes running past the motor mount shields. Having just done a top end refresh I cannot recall any high temperature source of radiant heat near the heater valve. If it's just generally hot in this area a shield probably won't help.

I am not saying I disagree but why would Porsche design it specifically for the 928?
More heat back there than you imagine - look at the effect of heat on the fuel lines.

Engine bay temperatures would be limited to roughly the coolant temperature except for exhaust components. They can radiate a great deal of heat, hence radiation shields to protect nearby components, but clearly the heater valve is not one of them. Convective heat transfer is possible, worst when there is no airflow - on shutdown, a transient condition. So here's my guess: perhaps the metal bit can absorb heat during this transient, and keep the diaphragm from going over a damage threshold on a hot day in Northern Africa, where Porsche liked to test cars. Maybe a test driver managed to cook one during a specific test of aggravated "heat soak".