If the shield was not needed, and did not serve a purpose, do you think Toyota/GM would spend the money putting one on the vehicle? Most replacement after market exhaust manifolds come with heat shields. You may have to check at your Toyota dealer for a shield for a 2004 Matrix. I do not know if it would be a direct fit but it should be close enough that you could make it work with some modification.
Here is link to check to give you idea of the cost. I do not know if the parts in the link are the ones you need but are for a 1.8L Matrix. This is the lower shield but below is a diagram with the exhaust shields and looks to have different shields for the upper.
http://www.toyotamarinparts.com/p/TOYOT ... 37100.html
There is also some shields listed here ay GM Parts Giant web site, look toward the bottom of the page:
https://www.gmpartsgiant.com/parts-list ... ifold.html