I notice some cars have black skirts that mount below the ground effects. It looks like a long L-shaped piece, similar shape to angle iron. I'm guessing it's just to hide the pinch welds and fender bolts. Did all cars get these? Mine doesn't have them, and I notice many others also don't have them.

 

Not factory. Probably a body reinforcing subframe added

 

If you’re talking about the black pieces under the side ground effects like the picture below - these are factory. Well at least on my 1984 Trans Am. I’m not sure what their purpose is. They’re typically missing from our cars. I assume they just get damaged from jacking up the car. The last picture shows the part numbers.

 

I've seen them on 1984 to 1990 Trans Am & 1987 to 1990 GTA. Probably were on 1989 & 1990 Firebird with W68 Sport Appearance Package. Can't remember seeing them on base Firebird or Formula, or 1991 & 1992 Firebird, Formula, Trans Am, or GTA.

If yours doesn't have them then they were probably removed at some point in the car's lifetime.

EDIT: ​Like @battmann I have no idea what they are for either

GM 10027611 Deflector, Rocker Panel Air GM 10027612 Deflector, Rocker Panel Air

 

I noticed them on this pic from the classifieds. I'm guessing they're just strictly cosmetic to hide the fender bolts.

 

Yikes on that picture! Crushing the area where the bolts are with a jack is another way of hiding them. LOL

 

Those are factory, and were used for aero. Those little things helped with airflow under the car at "elevated" speeds.
Another wind tunnel trick added by Pontiac.
Helped the Bonneville teams stay on the ground.
Have a magazine article someplace that mentions them

 

Looks like those parts were discontinued years ago, so I guess my car will continue to be without them. They probably got removed when it was repainted and they didn't bother to put them back on.

 

My '91 GTA has them.

 

I couldn't remember for sure if I had seen them on one or not.