I'm sure that it happened, quietly and often.
Even as recently as 1983 -
a 1983 article with the title "Early Management and Decision Making for the Treatment of Myelomeningocele". A medical team at Oklahoma Children's Memorial Hospital used a "quality of life" formula to decide whether to give parents of infants with spina bifida an "optimistic" or "pessimistic" prognosis. Parents given an optimistic prognosis were informed of all available treatments and urged to start them immediately. All parents so informed agreed to treatment. The parents of children given "pessimistic" prognoses were advised to forego intervention and treatment as the child would be too disabled to ever enjoy life even if they survived. Most parents, having been told this was in the best interest of their children, agreed. A very few did not. Here are the results in the authors' own words:
"The 'untreated survivor' has not been a significant problem in our experience. All 24 babies who have not been treated at all have died at an average of 37 days."
- Reference: Gross, R.H., et al. (1983). Early Management and Decision Making for the Treatment of Myelomeningocele. Pediatrics, 72 (4), 450-458.