It makes sense that ethnicity would have been recorded as well, but FAFAIK the Germans mostly used the data on religious identity (i.e. which persons were a member of which church community).
The point still stands; any kind of private information like ethnicity, religion, gender identity, sexuality - NONE of those are relevant to a government, and ANY of those can be used to identify and persecute people if there is a regime or law change aimed at any of those categories.
Tangentially related, it's why things like period trackers are dangerous too; the US has had a spate of anti-abortion laws, the data from those apps (if used) can be used to detect pregnancies and whether they have stopped prematurely. For example.
My reply wasn't meant to refute the parent's point. I wanted to clarify because in today's society ethnicity is a much more contentious data point than religion. Given the prevalence of race-related issues in the media it makes much more sense to a present-day mind to say "maybe we shouldn't be recording ethnicity" than to say "maybe we shouldn't record a person's religion".
*religious affiliation
It makes sense that ethnicity would have been recorded as well, but FAFAIK the Germans mostly used the data on religious identity (i.e. which persons were a member of which church community).