Giving horses slices of bread as a treat is common where I live (Australia). For many people it's as common as giving horses carrots and apples. Having said that, a horse may get what I refer to as snack amounts, one or two slices or up to 6 slices as a treat and, usually, no more in a day. The amount of grain in that amount of bread is so small compared to the grain intake of horses being hand fed grain. I feed a couple of my horses 4 one litre scoops of grain per day (in separate meals). Put into that perspective, the grain in bread is negligible.

I feed my horses bread as a treat, maybe once or twice a week and do so without any harm to their health and know of many people who do the same thing and use bread to help catch their horses. AFAIK there is no health issues for feeding snack amounts of  white, whole meal or multigrain bread for horses. It's certainly not something that should form the basis of a diet, and neither should apples or carrots either.

If I come across any more information on this I will edit and add it.