Often referred to as the 'barefoot' diet. But what is commonly recommended for barefeet is a diet that is suitable for horses which respond negatively to diets which are high in sugar/simple starches. The symptoms often show in the feet as laminitis/LGL/footiness/event lines etc. Because the above type of metabolic issue can have its symptoms masked by shoeing it is often not until the shoes come off that the owner/carer is aware that the horse has a problem.
So a low sugar/starch diet often referred to as the 'barefoot diet' has evolved.
There are variations on the theme but common elements include:
feed products which are low in sugar/simple starches
avoid bagged 'mixes'/cubes which include molasses, sugar or cane syrups
opt for feeding mostly hay, unfertilized meadow is a good choice
Thunderbrook.co.uk do a base mix which will be suitable for many
As the diet is safe for probably most equines then the low sugar/starch diet is often recommended 'blanket' style to all new barefooters. A sort of 'better to be prepared' approach.
However there are happy barefoot horses/ponies out there who eat all sorts of sugary rubbish and do just fine. So it is not 100% inevitable/essential to put a barefoot horse on a low sugar/starch diet. But it is a good idea and in the long run they will probably be better off even if they don't have metabolic issues now.
I would always recommend consulting an independent (of feed companies etc) equine nutritionist as each horse is an individual and what suits one horse may not suit another.