The bug

For a very, very long time, the horse jumping sound has mysteriously went unused as of today. When you jump while riding on a horse, only the landing sound is played. The horse jumping sound can only played when using the playsound command.

The playsound commands

 /playsound minecraft:entity.horse.jump player @s 

 /playsound minecraft:entity.skeleton_horse.jump_water player @s 

Not only the horse jumping sound is unused but the subtitle "Horse jumps" is also unused. All the "Music & Sound" settings are left untouched. And I'm pretty sure that anyone might have not noticed this.

How to reproduce

1. (Optional) Enable subtitles
2. Tame a horse
3. Jump
   → There is no sound for when horses jump, only the landing sound is heard. With subtitles on, the subtitle for the horse jumping sound is absent as well.

^{Note: This has been reported as MC-145437, but the reason why I created the issue is that the user themselves has been inactive as their last activity was in March 2019 with no response at all. I can confirm that this is still an issue as of the latest version (1.15.2) and the latest snapshot.}

I also attached the sound file itself here of what it sounds like when it used to play in game.