No path for water - update - take two
Sorry, I grabbed the wrong pic of rust to make my point. Good eye. My first pic was of the rear edge of my passenger door ( not the rear of the front fender as intended). Im embarrassed to admit I wasnt aware of the slots you described, I have them on my doors too.
The area I dont see a vent is better conveyed in the picture in this post. In the picture with the gray tape to the left of where a rubber plug was installed is the area of concern. As debris and water get past the cowl cushion, they accumulate below and behind the factory installed plug. On my car the passenger side capture nut failed, but the fender sheet metal and brace were fine. On the drivers side the nut was fine, while the sheet metal and brace rusted out.
I believe rust in this area is common. If you have the 2018 OPGI book handy, look at the lower front patch panel on page 217. It conveys better in a photo what I tried the describe in words.
That is one good looking GTO you have. No doubt the previous owner wishes they still owned it.
P.S. My car left the factory White. I painted it Black in the mid 70s. It will be Red Metallic in a few months.