Quick Answer:  Yes grazing muzzles work.  They restrict grazing and the amount of grass a horse can eat thus reducing the amount of sugar they are consuming and this will help prevent Laminitis.

There are several ways to reduce the amount of grass a horse can eat, strip grazing can work by just allowing a tiny amount of new grass each day.  Or if you are lucky, a track system with no grass is the gold standard, the horses' diets can be controlled very precisely and laminitis kept at bay.

However we do not all have access to these options and so a grazing muzzle becomes necessary.  There are many to choose from, some favourites are light weight and flexible but they can break too easily and that becomes expensive - they are not cheap - plus horses soon learn to eat out of the sides.

Another popular one is made from hard plastic and can be very hard on teeth, this kind also rubs easily.

Then there is the LiteBite Horse Muzzle, created by Celie as her pony Skye would not eat with any of the other muzzles on the market, so came the LiteBite. It is soft and flexible, very strong with a unique grazing slot, not a little round hole, allowing a normal bite even if the width of the slot restricts the grass taken in.  There is no eating out of the sides either.

Definitely a problem solver and well worth a try, full money back guarantee if it doesn't suit.