From Milton to Baloubet du Rouet - Iconic Wins at Gothenburg |Longines FEI Jumping World Cup™ Finals 

I can't speak for all, but they do get retired and either go to stud or spend the rest of their days either with the riders who last rode them/competed them (some horses go to different riders in their career) or with their owners. Many are owned by syndicates now so they often retire with the rider of the syndicate ☺

As far as I know, they all go off to really good retirement ranches or stables and live amazing lives, and yes no slaughterhouse.

i mean all the horses from the 80's/early 90's were between 10 and 16 years old, so they're all most likely dead now