Well I have some evidence that there is truth to this 'so-called' myth. One of the ploughmen (aged 93yrs) I was speaking to about my Clydesdale told me that a lad he was training up wore the steel toe capped boots. They were training a couple of Shires to pull a cart. The lad got his foot in the way of one of the Shires who was also shod, and the weight of horse, hoof and horseshoe caused the steel toe cap to collapse around the lad's middle toe. He ended up with a fracture of his foot and big toe and the middle toe which was caught under the steel cap, had to be amputated. At first I thought this was a 'country myth' but I was corrected when the old ploughman told me the lad in question was his son. He said he never wore steel toe caps again around heavy horses and neither did any of the other 'fellas'. Just to clarify that Heavy Horses in the UK mean your big, giant horses such as Shires, Clydes, Percherons many of which weigh in around a ton and not heavy cobs (draft type).
The accident occurred while the ploughman's son was a young man, which given his age, would imply that the accident occurred several decades ago. A steel toed boot manufactured decades ago is not comparable to one manufactured today, and so the perceived failure of of the former does not negate the benefit of the later. Modern safety certifications and such ensure that fact.
A weight sufficient to crush a steel cap is easily enough to shatter bones severely enough that amputation is necessary, because a bone broken into dozens of pieces will never effectively heal. The weight absorbed by the metal surely lessened the force that impacted the foot, and in turn the damage that done by it.
I've heard many stories by older fellows, from, 'seat belts do more harm than good,' to, 'cigarettes don't harm your health,' and although I nod in agreement, I know they're just charming, yet stubborn, old men set in their ways.
It's your choice to wear steel toes or not, but every workplace safety organization worldwide requires steel toes for working around heavy weight objects, including weights equivalent to hundreds of drafts. Those organizations have made the decision to require steel toes based on thorough study of how to protect to the foot. To insist steel toes present a hazard in spite of that fact, well...