In our experience in research on a huge scale in UK government, this is not true in our context. People expect to have to fill in a field if it's asked. Reducing optional fields to a minimum and marking them optional has been the best approach.
Makes sense in context. Perhaps same with applications (job, uni) where expectation is that everything is required and users want to give as much info as possible. Other forms (e.g. mailing list) don’t have this expectation and some seek to give as little information as possible.