“Sound” is a spectrum. I would bet that most of the horse population, racing or just hanging out in the pasture, would not meet a strict definition of “sound” that means there are no abnormalities or inflamed areas in any structural part of the horse.
The judgment calls for “racing sound”: can this horse do what is asked of him to the best of his natural ability without experiencing pain or catastrophic injury? Veterinary diagnostics have advanced to the point where early detection of conditions or injuries to a horse that is NQR mean that horse gets time off. Or retirement, before a chronic condition results in the unthinkable. And people gripe when this happens - “there goes my bet”, or “retired too early, they could have rested him/her”. It’s a no-win situation for everyone.
True that we don’t see the Iron Horses of years past, but I think that’s a good thing. Forego was just lucky not to have broken down and, IMO, should have retired long before he did.
It’s not just racing - it’s any performance horse. You ask yourself “can I ride today and ask questions of this horse without having to worry that I will cause him pain”.