Sadly, it states in the rule book that they don’t have to be sound for equitation.
EQ102 Judging
- SOUNDNESS. Unsoundness does not penalize a competitor unless it is sufficiently severe to impair the required performance. In such cases, the imposition of a penalty is at the judge’s discretion. (Exception: Hunter/Jumper Seat Equitation classes held at Hunter Jumper Competitions where, in a case of unsoundness sufficiently severe to impair the required performance, the judge(s) must eliminate the competitor from that class and inform the competition manager, who in conjunction with the Official Veterinarian and the Steward, will evaluate disqualifying the horse from further participation in the competition.)
Bolding mine. I guess this is where “serviceably sound” comes from, although I tend to keep that label for lesson horses and novice children’s/adult horses…