The only horse out of 3 we have on a supplement is our yearling. I have him on a mare & foal vitamin/mineral supplement because he is growing.
The older horses may get a bit of Strategy or other grain, but it is more as a treat than something to provide much in the way of calories or nutrients. When my mare was pregnant I did give her a ration balancer (Purina Enrich 12).
Maybe I am poor or live in the wrong neck of the woods, but most people around here just feed hay and water and my guys are pretty spoiled getting Strategy or grain as a treat with their hay.
No one else I know even gives a vitamin/mineral supplement to their growing horses.