How about an email client that uses Glide, the 3DFX 3d rendering library?
Specifically targeting the voodoo 2. (I know it's PCI, but there are PCI->PCIe adapters).
The OS can't screenshot it because the OS has no idea what pixels are there!
on the plus side, it's physically impossible for any AI bullshit to screenshot it, unless you accidentally attach a VGA capture device or point a camera at the monitor