FEI rule I could find read:
GENERAL RULES
FEI CODE OF CONDUCT FOR THE WELFARE OF THE HORSE
2. Fitness to compete:
d) Surgical procedures
Any surgical procedures that threaten a competing Horse’s welfare or the safety of other Horses and/or Athletes must not be allowed.
+*
DRESSAGE RULES
ARTICLE 416 THE IMPULSION / THE SUBMISSION
2.1. Putting out the tongue, keeping it above the bit or drawing it up altogether, as well as grinding the teeth or agitation of the tail, are mostly signs of nervousness, tension or resistance on the part of the Horse and must be taken into account by the Judges in their marks for every movement concerned, as well as in the collective mark for “submission”.
ARTICLE 428 SADDLERY
6. False tails/tail extensions are permitted only with the prior permission of the FEI. Requests for such permission should be directed to the FEI Dressage department accompanied by photographs and a veterinary certificate. False tails may not contain any metal parts, (except for hooks and eyelets), or extra added weight.
If you get a ruling from the FEI that a horse that at some point in its’ life had surgery on the tail cannot compete, I would be interested to know.
The USE(F) divisions Dressage, Jumping, Combined Driving, Reining and Endurance do not address tail alteration other than the welfare statement matching the FEI and that in Dressage tail agitation or wringing are faulty.
Several breed divisions have rules, but the Disciplines do not.
I can see where tail carriage would affect judging a Dressage horse, or a Reining horse; not so much in the others.
Realize I do NOT like surgically altered tails; I think they ought to be relegated to history.
That doesn’t change whether or not showing a horse in Sport divisions is legal.
And doesn’t change that buyers also do not like them.