@dalias @JessTheUnstill We already have something like that in France: the government gives your employer the percentage of income that must be paid. It's still displayed on the paystub.

In practice, it means your employer can infer personal details from that percentage alone, and track it over time. If it's high, you're probably single. If it's medium, you're probably in a small family. If it's low, you're probably in a big family. If it's higher than high, you're earning more money via another gig...

There is an option to give your employer a "standard" percentage and pay/refund the rest, but that percentage is between medium and high, so penalizes everyone but singles, so almost no one bothers with it.