[quitit]

A census is only useful if the participants can fill out the paperwork honestly. I actually don't understand the people who don't see this being a problem, nor why they would believe layered privacy protections aren't useful. It would seem irresponsible to release accurate or detailed census data to the general public.

To use an example that I'm aware of: in the USA (but likely exists in other places), there are groups of individuals which seek out and target interracial couples. These couples are then harassed and violence is not uncommon.

Approaches such as Differential Privacy exist to address this specific privacy weakness. The act of fuzzying the data this way is an accepted method for the data to only be useful for its intended purpose. As you and others have noted it's trivial to build up various use cases, from the commercially annoying to the dangerous: the assumption that individuals and companies are never going to try to exploit this data or break the law is perhaps dangerously naive.

[lupire]

The problem is that the census should not be collecting this information in the first place, because it undermines the mission.

[quitit]

I'm not a statistician so I can't possibly get into a discussion about what is worthwhile information for collection. I am aware that a lot of planning goes into such questions to avoid potential abuse (even by the government), the goal being to ask the minimum needed to provide governance while also providing a historically meaningful snapshot. I don't think there is a strong argument against having a census, it's a more privacy preserving approach to planning than other forms of data collection such as mandatory registers or combining existing government databases - medical, births/deaths/marriages, automotive, postal, taxation, education and so on (in this example the resulting database has too much information about the population, governments are mindful to keep these databases separate on purpose.)

Back to your point though: Arguably a lot of needed and seemingly benign information can still be problematic. As one commenter noted, merely indicating childrens' presence can be a problem, while such information is clearly beneficial for governance.

Ultimately protections should be in place regardless of the kinds of information that is sought, since one can't foresee all potential forms of abuse such data collection can bring, nor how such information can be merged with additional data sources to reveal more accurate profiles (as is the contemporary issue of online privacy.)