The field doesn't do anything by itself. There is zero harm inflicted on people using systemd. There are probably lots of features of systemd that you don't want or use and the entire negative effect that you suffer is a few megabytes less free storage space.
The only way the field would be used is if a person decided to use a different piece of software that wants a birthdate. If they don't choose to install such a program then the field is no more a danger than the realName or location fields. They have scary sounding labels but do absolutely nothing unless the user chooses to use them.