I have been actively researching my ancestry for the past 7 years. My paternal grandparents were immigrants who entered this country in 1890s as teen-agers to escape an oppressive Europe where there were no jobs, no money, no food. The locale where my grandparents immigrated from lost 27,000 young men, the majority came to America with some going to the Australian gold fields and a handful to Norway.
For every year from 1890 to 1950, the Federal Census asked whether a person was an alien. This was a Yes/No question followed by the year of entry. If naturalized, what year.
Keep in mind, the previous census to 1950 was 1940 just prior to WWII. I found in my research that many young men who entered this country as children and then either voluntarily enlisted or were drafted into military service where given automatic citizenship in the camps where they were sent for military training. There an immigration agent granted these young immigrant men American citizenship. It did not matter if these young me were stationed at Camp Kearney in San Diego (CA), Fort Ord in Monterey (CA), or Fort Greeley on Kodiak Island (AK), they were granted automatic citizenship for enlisting to fight in our armed forces.
I surmise 1950 was the last year immigration status was queried on the census form because the vast immigration tide had ended plus, when the 1960 census arrived, there was civil unrest in this county regarding rights which led to the 1963 Civil Rights Act. That is my theory.