My car’s heat shield has had degradation of the insulation that goes between it and my engine/exhaust. The Subaru dealer I take it to for regular maintenance has told me a new one is what I need in order for the rattling noise to go away. Is this a common problem for a 10-year old Subaru? It seems like an expensive fix of around $400 to replace it!
I would say it is common. Many of that vintage had heat shields with spot welds that would give and then the thing would rattle like a marble in a can. I suggest you try some local shops to see of the price can be any better.
I would suggest that the OP visit some local muffler/exhaust shops.