The Supreme Court seemed willing Tuesday to let the Trump administration add a question about citizenship to the 2020 census form that goes to every U.S. household, despite claims from populous states that it would actually make the count less accurate. To whether illegals will not send the census form back or lie on it or be honest is an open question. The information is protected from law enforcement / not shared with law enforcement and so there is no reason for illegals to lie but it is understandable that it is in their nature to not trust government.
The Supreme Court will decide the case by late June.
In 2010 Census Workers went to around 48 million homes either because the census form was not returned or as a follow up to the form. As I learned the hard way one year, it is best to fill the form out and send it back. Yes. It. Is.
Be aware there are scam artists who pretend to be census workers so always make sure you are dealing with someone who actually is a census worker.