"The so called static shield in the engine-bay lid seems to be nothing more than a ground-plane, grounded to the chasis. Is that all it is?"
Yep. Most electronics that is inside plastic cases has a metal RF shield underneath. I guess the decklid on the Fiero could be considered a "plastic case".
"Is it there only for protection of the radio, or...?"
It's there to prevent RF interference from outside the car from interfering with the signal wires going to the ECM, and vice versa. (So it doesn't interfere with your radio, etc).
"Does it also somehow shield the ECM?"
It shields the wires from the ECM, I think the metal case that the ECM is in shields it more than anything else.
"What happens if it is not grounded?"
That one I'm not sure about. However, RF shielding is usually grounded so I would assume it is more effective this way.
"Is it some exotic material, or just steel/aluminum/or copper?"
I have no idea what it's actually made of, but it's probably something fairly normal like that. No need for anything exotic.
One interesting thing to note is that the 88's don't have this. Their decklids were specially made by impregnating the plastic with some type of metal/alloy to "build in" the RF shielding to the plastic. I believe Pontiac won some kind of industry design award for this.
From: Sketch