It's 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.

Lie by omission here. Clearly the reason for this being added is AV. Imagine it was a "sexualOrientation" or "ethnicBackground" field that just got added because $government suddenly made that sort of thing a hot topic... "oh it's just part of the JSON schema, it's not up to us to decide how people use those fields! Fuck off. systemd should have no knowledge of any of that. It's irrelevant to the operation of the system.