I think the question will not be on the census because there are other more reliable ways to find this information. Questioning a respondent is likely the most unreliable way to gather statistics. People lie on surveys all the time. Some do it for a reason. Others do it for no reason, just as a joke.
With any person in America, if the government has a name, address, date of birth, and other information provided by the census, it is a relatively simple process to determine whether or not they are naturalize or a citizen.
I think Trump backed down because after causing a huge shit-storm, level headed people finally convinced him that, one: it is not necessary, and two: it is unreliable garbage information anyway.