I'll start off my comment with something everyone can agree on: the age verification laws absolutely sucks. It's a surveillance law masquerading as a means of "protecting" children. It also completely undermines free and open source projects, and therefore, protected speech. The fact systemd had to add a BirthDate field is unfortunate, however, I would blame the lawmakers for creating the law that the developers of systemd now have to comply with.
I'm okay with the implementation. It is an optional (meaning you have to add it yourself) field which only specifies the date of birth. It doesn't seem to be at all invasive, nor does it attempt to "verify" it at the moment. Granted, anything is possible, but I don't think there's a good enough reason why systemd would EVER feel the need to add age verification. Before you say anything regarding corporations, please answer this: why would a corporation add age verification to a system manager their servers depend on? How will that profit them?
I get why people are angry, but I think this anger should be funneled towards the lawmakers pulling off nonsense like this. Fight those who are actively trying to take your rights away. Bullying software developers for complying to international laws will lead to nothing but hate.