I finally had some free time to mess around with left4Dead 2. I came across this while I was setting up a center-screen-HUD fix.
Undeniable proof that parts of the HUD are behaving in a vert- manner:
Normal:
Triplehead:
Normally, those floating pop-up messages are confined to a small box in the middle of the screen. This invisible box that limits how far these pop-ups can move from the center of the screen is a fixed aspect ratio, but it expands as the screen gets wider. Once the screen gets wide enough, the top and bottom of this boundary box go off the top and bottom of the screen (cropping occurs).
Icons within this boundary box are now allowed to trail off the top and bottom of the screen, meaning they get cropped as well.
Here's what that invisible boundary box looks like at 16:10
And here's what it looks like at 48:10
As you can see, it's quite obviously being cropped, allowing icons to run off the top and bottom of the screen. It appears that certain HUD elements (this boundary box, the crosshair, the pop-up icons themselves) only look at the horizontal width of the screen when they determine their size, and just assume the screen will be tall enough to fit everything.