[gzread]

The existence of the user birthday field is mass surveillance, but the GECOS field, which contains your full name and street address, and the username, which often contains parts of your name and is mandatory, somehow are not. Full access to your home directory, which includes all of your passwords and usernames and confidential data, also somehow is not an invasion of privacy.

[spaceheeder]

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.

[gzread]

Putting your birthday in the birthday field is completely optional.