I hope, probably with excessive optimism in this case, that even our incredibly corrupt and inept neo-fascist Supreme Court, while invalidating a part of the Constitution, might balk at also making a judgment doubly unconstitutional by being retroactive.
You know, when you think about it before the 14th amendment, there was no provision that prevented the children of undocumented immigrants from being citizens. No documentation was required. The amendment was passed to undo the monstrous deprivation of citizenship to slaves and their descendants, not to grant anything to anyone else, because I'm pretty sure one would find that IT WAS ALREADY GRANTED! Before that amendment a non-enslaved person born in the United States had always been presumed by default to be a citizen. Not that this would prevent a new law from setting new boundaries, but it ought to be remembered, I think. Because the argument for nullifying part of the 14th amendment would likely be based on the presumption that the change was not intended to apply to undocumented babies, but the reason for that is that it did not have to. An argument on intent really should address the entire Constitution and its presumed intent.
I don't think a SC judgment would result in a new law on birthright citizenship without legislation redefining it. The amendment states that any law diminishing birthright citizenship would be illegal, but I don't think changing that alone would make a new law. And the Constitution explicitly forbids ex post facto laws, and that's in the main body, not the disputed amendment. So though I can imagine the SC might well be stupid and evil enough to corrupt the 14th Amendment and unleash the panoply of repressions and inconveniences and errors and sins that will almost certainly follow, the Constitution ought, pretty obviously, to limit such legislation to births occurring after the legislation is passed. Until that occurs, the law of the land still applies. Or ought to. Not that it will, because I think we're basically ◊◊◊◊◊◊.. But who knows, maybe there's a pony under there yet....