Since this is sd-userdbd we are talking about unless the used backend provides the value it is unset by default. And if you manage your home directory using sd-homed unless you explicitly set it it is also unset by default.
> It's entirely optional, I get that. I could 'just' not set anything.
Why? Telemetry, mainly. I'd rather attestation [or whatever intends to use this] fail and make it apparently deliberate.
The failure/assumption of under-13 or whatever, as a result of manipulation, is fine. I'm not actually trying to solution something though, jeez.
I find it more compelling to say, for instance, "x% of our users have chosen not to share their information"... rather than "y% have not set it". This category would almost surely be about as 'useful' (useless) as the 'do not track' header... and a concern for something other than systemd or even the portal (to a degree).