So it's the legistlation's fault again, why aren't you mad at them, why only systemd? In other jurisdictions you don't have to use this field. And I don't see anything in the PR about the verifability of the date. It's just an optional number it stores in a db, offline.
Programmers have to become lawyers now?
Also a lot other projects has a birthday field, e.g. last time I worked with was LDAP: https://ldapwiki.com/wiki/Wiki.jsp?page=Birthdate I guess it's there since the 90s.