Klippel–Feil syndrome is rare and it likely doesn't describe one single disorder. Instead, it's more of a catch-all, a name for a variety of conditions that all share one common feature — being born with some of the vertebrae in the neck fused together.
Besides that, Klippel-Feil syndrome is pretty diverse. It's associated with a wide variety of birth defects that not everyone with the syndrome has. So it's hard to say what an absolute outcome for Klippel-Feil would be. But, for one man who lived 4,000 years ago in what is now northern Vietnam, Klippel-Feil syndrome likely meant complete paralysis of the lower half of his body. There's a good chance his arms were at least partly paralyzed, as well. His head would have been torqued to the right. It was probably hard for him to chew. Basically, he couldn't have easily kept himself alive with no help
And yet, this man — known as Burial 9 — lived into adulthood. Discovered in 2009, he is only one of a collection of prehistoric burials demonstrating that, even while living under harsh conditions, our ancestors went out of their way to care for people who couldn't care for themselves and make space in the community for people who had to live differently than the norm. In the New York Times, James Gorman writes about this archaeology of compassion:
Among archaeological finds, she said, she knows "about 30 cases in which the disease or pathology was so severe, they must have had care in order to survive." And she said there are certainly more such cases to be described. "I am totally confident that there are almost any number of case studies where direct support or accommodation was necessary."
Such cases include at least one Neanderthal, Shanidar 1, from a site in Iraq, dating to 45,000 years ago, who died around age 50 with one arm amputated, loss of vision in one eye and other injuries. Another is Windover boy from about 7,500 years ago, found in Florida, who had a severe congenital spinal malformation known as spina bifida, and lived to around age 15. D. N. Dickel and G. H. Doran, from Florida State University wrote the original paper on the case in 1989, and they concluded that contrary to popular stereotypes of prehistoric people, "under some conditions life 7,500 years ago included an ability and willingness to help and sustain the chronically ill and handicapped."
In another well-known case, the skeleton of a teenage boy, Romito 2, found at a site in Italy in the 1980s, and dating to 10,000 years ago, showed a form of severe dwarfism that left the boy with very short arms. His people were nomadic and they lived by hunting and gathering. He didn't need nursing care, but the group would have had to accept that he couldn't run at the same pace or participate in hunting in the same way others did.