What is the name of the plastic cover that bolts to the sub-frame under the engine?

Do you have a picture of it?

Do you mean the one that extends forward to the front bumper? I would call that a valence panel.

The one that goes under the radiator and front of the engine. It is a black plastic panel.

Valance panel.

i was thinking skid plate

Skid plate is made of metal, is further back and protects the oil pan.

**621132 DEFLECTOR ENG SPH SHLD AIR.. I looked this up on the gm website this is what they call it. The website is gmpartseast.com. Its in section 9 body mounting Component 17. 1999 Pontiac Bonneville - GM09017101 A/C Refrigeration System L362HX69 That link will take you right to the picture of it.

Or in my world, its called an annoyance shield and has been removed years ago.

Yes i know a lot of stuff is like that with you lol