Hmm, I seem to have confused myself...

Depends on the sport. Dressage any plugging of the ears is against the rules, not 100% sure on rules for other sports I'm afraid!
 
Dressage no - horse supposed to be obedient to rider and ignore distractions (includes eventing dressage).

Jumping - fine. Purpose is to get over the fences, doesn't matter if horse is "obedient" - though obviously it helps!
 
It happens all the time in Showing too.

In racing you have to declare your horse with them if they need to race in them, if they just need them for the preliminaries and get taken out before the race starts then there is no need to declare them.
 
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