[lonelappde]

Politifact couldn't find evidence of prosecution for lying or refusing since 1970.

https://www.politifact.com/factchecks/2014/jan/09/us-census-...

[crooked-v]

From what I understand, the illegality is mostly used as (a) a tool to increase the baseline number of responses, and (b) to by consequence make it illegal for anyone else to force someone to not complete the survey for some reason.