This change makes horses less and less useful. In my area there are thin thicket lines surrounding almost all biomes, making a kind of impenetrable Chinese wall for horses. It's a cage. It's a ridiculous change that makes no sense at all. Horses were only used for foraging, looking for clay and go to fishing zones fast. Now they don't even have that since the damn thicket is everywhere surrounding areas.
If you want to keep a penalty on horses there are less aggressive ways. For example, when they pass through a thicket area, they lose 30% of pony power and go 200% slower, but at least they can pass.