Is heat shield needed?

It's there for a reason but if you don't want to replace it shouldn't be a big problem depending on where it is and what it does

located rear driver side, under trunk

That heat shield protects the interior from getting VERY hot from the catalytic converter. As there's no ventilation in the trunk, it could catch fire. Tell that idiot who removed it that Toyota wouldn't have installed a heat shield if they didn't think it was necessary, and have him put it back, at his cost... Then never visit him again!!!!!!

Exactly, it was put there for a reason! Sounds like it's near the gas tank. Looking at a diagram of your exhaust system, you have two mufflers back there. One on each side. Have the heat shield replaced and start going to another mechanic! Jim

Time to find a new mechanic....