It's interesting to note how quickly the neck curtain and spinal pad disappeared from use when the transmission of malaria by mosquitoes was proven (around 1900).
Prior to ~1900, there was medical controversy and confusion concerning heat stroke, sun exposure, tropical fevers, and malaria. The median survival of British troops in West Africa was said to be about 18 months, so it's not surprising that removing the sun helmet during the day was viewed as courting disaster from tropical fevers,
The cholera belt disappeared a little earlier when the bacterial cause of cholera was identified (1883).
Of course most of South Africa is malaria-free and it's a healthy climate; If I recall correctly, neck curtains don't feature in photos of British troops in SA much after mid-1900, and mostly the pictures date before Jan 1900.