I don't know the answer to that myself.
Have you tried looking at the difference between AbstractHorse in 1.12 and EntityHorse in 1.10.2 to see how the vertical parameter has changed the motion calculations?
What Draco means is that there is no specific event related to that method, but there are several tick and update events that you could use to have your own code to check the block the entity is on. For example, you can using the living entity update event and check which block is under the entity.