You also can't have automatic voting registration until you keep a list of citizens, and one doesn't exist. The census doesn't ask people's citizenship status.
About your last point: do you also mean that local census bureau offices do not keep track of citizenship status?
No. Trump tried to add this as a question to the census and it was blocked by the Democrats/courts.
I know about the 2020 Census question thing, but I thought that local offices had citizenship lists.
If they did, then Trump would've never needed to try adding a citizenship question.
I mean, I have just found out that the United States have no resident registration.
Which I should probably have already known given that infra-Census population estimates are apparently always based on household surveys like the ACS.
Or by looking at StateBoiler's updated post, but I read it before it was updated.
I am simply so used to the fact that Italy has compulsory resident registration, which keeps also track of citizenship status. And of course, register offices have this information independently of the Census.