No, a horse won't founder off oats, I agree. Oats are high fiber and are readily digest. Here in the USA, many people have gotten away from feeding oats in favor of mixed or pelleted horse feeds, which contain many ingredients that aren't natural to a horse's system. Non-symptomatic laminitis can present with any high sugar/starch feeding program. The horse can seem fine, but just be a bit short strided or grumpy on certain surfaces. You'd think it was more likely a muscle, chiropractic, or other soreness issue, but in some cases it's actually low-grade laminitis, and a change in diet clears up all of the "symptoms". It's very interesting research.