Most consistently successful generational breeding programs utilize some degree of inbreeding, unlike most breeders producing F1 (sometimes F2) sport horses as their final product. The important thing is knowing how to balance the inbreeding with outcrossing to manage the overall level of inbreeding in the breeder's herd. Outcrossing can, in one generation, cut the inbreeding coefficient of a very inbred parent to an exceptable level. In other words, inbreeding is more about the whole herd than the individual. As long as strict selection is at work and there is an outcross available for every horse in the breeding herd, inbreeding is a legitimate tool in the generational breeder's toolbox.
In racehorses, it's often the subject of many discussions!
I read this article about it: https://www.globalanimal.org/2011/12/01/inbreed-horses-race-to-ruin/
Here is an extract “In the 1960s it was usual for each stallion to cover 40-50 mares per season, in the mid-1990s this number jumped to 150+.
Nowadays, high quality stallions are also “shuttled” around the world to cover mares, for example, being sent to the southern hemisphere to breed with mares during the quiet season for breeding in the northern hemisphere.
This in part is to meet the modern demand for producing yearlings that sell for high prices at auction rather than the previous breeding goal of producing superior racehorses.
Overall that means fewer stallions are siring a greater proportion of offspring.
The current trend toward greater inbreeding is “worrisome”."