This seems’s like an issue created by congress. the constitution only requires a headcount by state. Maybe they should use another mechanism to collect demographic data. Since the concern is not about representation, but allocation, tax returns seem like an obvious alternative and they are already private and collected at a much more granular level.
> tax returns seem like an obvious alternative and they are already private and collected at a much more granular level.
Plenty of people don’t file tax returns (legally even, there’s a minimum income threshold).
I don't think the question "Has this person given birth to any children in the past 12 months?" would look good on a tax return.