"The systemd project merged a pull request adding a new birthDate field to the JSON user records managed by userdb in response to the age verification laws of California, Colorado, and Brazil," reports the blog It's FOSS.

They note that the field "can only be set by administrators, not by users themselves" — it's the same record that already holds metadata like realName, emailAddress, and location:

Lennart Poettering, the creator of systemd, has clarified that this change is "an optional field in the userdb JSON object. It's not a policy engine, not an API for apps. We just define the field, so that it's standardized iff people want to store the date there, but it's entirely optional. " ...