I agree with you that Trump is on to something. As I posted before, my issue is that the POTUS cannot end birthright citizenship for children born to illegals via executive order. He can certainly try but it won't stand.
Which I think DeeJay has addressed - that he didn't say he would change it with a wave of his hand but that he would, with executive order, make the concept clear the legal authorities.
Brennan claimed his authority for his footnote came from a Clement Lincoln Bouve, a lawyer in the Copyright Office. He wrote this treatise on what the proper procedures and rules regarding expelling people from the country (my guess was at the time he wrote it -just around the first World War - the concept of being in the nation illegally wasn't fully formed, even as citizenship - WAS). Since I've only ever read descriptions without reading its text, I can only surmise that birthright citizenship granted to illegal immigrants was NOT the primary purpose of his treatise.
In any case, he was not a judge, or an elected official. Just some lawyer with an opinion. And a SCOTUS judge decided, let's just insert HIS opinion into my footnote.
So to answer your question, I believe the
practice prior to 1982 was to expel illegal immigrants ALONG with their progeny. With a wave of HIS hand, Brennan was able to declare them citizens without properly arguing the case.
You have to admit, it IS kind of weird that a nation that holds jus sanginus - that a child born to say, French or Italian citizens is automatically Italian or French - but that if they were here illegally, they'd be American, too.