[tripletao]

I think those optoisolators are indeed sold mostly for switching power supplies. That's probably why someone cared enough about aging to write an app note, since the ambient temperature is high there and the exact CTR matters more when it's in that analog feedback loop. I've also seen them for digital inputs in industrial control systems, where speeds are slow and the wires might be coming from far away on a noisy ground.

That said, I believe optical isolation is typical for these "data diode" applications, even between two computers in the same rack. I don't think it provides any security benefit, but it's cheap and customers expect it; so there's no commercial incentive to do anything else.