This is one of the most sensible comments in the thread. The law is the problem. This is something which should have been self regulated by websites themselves, but Meta lobbied for laws like this so they wouldn't have to police it. The law making this mandatory for everyone when this should be a parental control is the issue.

This is a law that companies are required to implement or stop making business in the states enforcing that law.

You probably feel that companies should just stop doing business in those states "to show them". Sadly a lot of profitable Linux companies that fund Linux development disagree with your high morals. They want to continue doing business there.

Adding that field help those company comply with the law and doesn't hurt you in anyway except maybe taking few bytes in your disk drive.

Even if the field is not added, those companies would come up with another place to store date of birth or even use systemd fork.

Its not like they will say since we can't store date of birth in systemd's user model then we'll have to abandon this project and close our branches in those states instead.