I will revolt, once this tries to attack my privacy.

I’m fine, as long it complies technical requirements that applications will implement sooner or later, while preserving my privacy by simply defaulting to 1900-01-01 or something.

The systemd change ‘just’ adds a birthday field to the user data, where you could store (or don’t) the users birthday, that then could be used by other applications to request an age bracket.

The Arch-change doesn’t effect real arch Linux. It modifies the archinstall script (so, irrelevant, if you install according to Wiki) to ask the user for their birthday during installation and stores it for systemd.