Correct, because putting additional data in GECOS is completely optional. This isn't about having your email client know your birthday, it's about phoning home

Correct, because putting additional data in GECOS is completely optional. This isn't about having your email client know your birthday, it's about phoning home to gate functionality. Verifiability, attestibility, all the loopholes that you think make this harmless are the croaks of a frog that is unjustifiably confident that the water in the pot won't continue getting hotter.