I don’t think that Trump has the power to undo Birthright citizenship by an EO. I don’t think he will even try. He might be trying to force a discussion on the topic or he might be trying to make immigration more important in the midterm elections. I think it is bluster. As for it being a common practice, not really. There are 30 countries that have birthright citizenship. They are almost all in North and South America. There are 193 countries on our planet, so we are definitely in a minority.