That got me curious about how Puerto Ricans get citizenship, so I read the Foraker Act of 1900 and the Jones-Shafroth Act of 1917, and the mentions of citizens in the current PR Constitution.
It appears that their rules for citizenship refer back to the Foraker Act. Anybody who was previously a subject of Spain, and did not make an oath to retain their Spanish allegiance, became a US citizen.
The Jones-Shafroth Act additionally provided that anybody not owing allegiance to any other nation and are permanently domiciled in PR would be US citizens. It also specified that anybody born to an alien parent and was permanently residing in PR would have a window of opportunity to gain US citizenship when the Act took force (6 months after the Act for adults; for minors, 1 year after reaching majority).
These and their children were granted birthright US citizenship.
It seems to me that most current PRs would have nothing to fear if children of illegal migrants or visa holders were ruled to be non-citizens. As long as their parents were here lawfully and permanently, the children would continue to have US citizenship.
The US Congress would be Contitutionally barred from writing an ex-post-facto (i.e. retroactive) law stripping citizenship from those already explicitly granted citizenship. That’s covered under the Due Process clause (another part of 14A) and Constitution Art. I sec. 9 regarding ex post facto laws.
The conservative push is only to exclude children of those unlawfully here and children of those in the country on a temporary visitor basis. From my reading, neither of those situations were ever included explicitly in the Foraker Act, the Jones-Shaffroth Act, or the current PR Constitution.