Both the Citizenship Clause of the 14th Amendment, and section 301(a) of the Immigration and Nationality Act (8 USC 1401(a)) grant citizenship to people born in the United States only if they were born "subject to the jurisdiction thereof" (i.e. of the United States). If someone were to successfully argue that birthright citizenship does not apply to a certain group of people, because they were not born "subject to the jurisdiction [of the United States]" within the meaning of both the Citizenship Clause of the 14th Amendment, and the Immigration and Nationality Act, and that we have been interpreting it wrong all along, then presumably the decision can retroactively apply to people who were already born, because this is an interpretation of the Constitution and laws that already existed at the time of their birth. The people would be deemed to have never been US citizens in the first place, and any documents verifying citizenship they may have been issued would be deemed to have been issued "in error". (This is similar to how, if someone's naturalization is later found to have been fraudulent, or when someone born in the US is later found to have been born to a diplomat like Hoda Muthana, they are legally considered to have never been US citizens at all, even if they have previously been issued US passports.) In this case, since they are considered to have never been citizens, there is no "loss of citizenship", and the Supreme Court precedents on loss of US citizenship do not apply.
If the court were to hypothetically limit its decision to apply prospectively to people born after the decision, then I believe that the precedents against loss of US citizenship would protect people born before the decision from having their citizenship involuntarily taken away by a future Act of Congress or the Executive Branch, because they would be considered to have been "born or naturalized in the United States, and subject to the jurisdiction thereof" within the meaning of the Citizenship Clause of the 14th Amendment. (At least those people who were born in the 50 states or DC. People born in unincorporated territories might not be covered.)
Another possibility is the court might rule that a certain group of people were not born "subject to the jurisdiction [of the United States]" for the purposes of the 14th Amendment and the Immigration and Nationality Act, and this applies to people who were already born, but as an interim remedy, in light of longstanding interpretation, allow people who were already born to keep their citizenship, as if provided by statute, unless and until Congress decides otherwise. Basically, this would remove the Constitutional protection against a future Act of Congress removing their citizenship involuntarily, because the US Supreme Court clarified in Rogers v. Bellei (1971) that precedents on loss of US citizenship like Afroyim v. Rusk (1967) only apply to those who were "born or naturalized in the United States, and subject to the jurisdiction thereof" within the meaning of the Citizenship Clause of the 14th Amendment. (Bellei was a US citizen from birth, born abroad, and therefore was not "born or naturalized in the United States". His citizenship was only granted by an Act of Congress, and the court ruled in Rogers v. Bellei that it was constitutional for Congress to take away his citizenship involuntarily, under certain circumstances.)
Another case I can imagine is if the court rules that a certain group of people were not born "subject to the jurisdiction [of the United States]" for the purposes of the 14th Amendment and the Immigration and Nationality Act, and this applies to people who were already born, but Congress subsequently enacts a law to grant citizenship back to some or all of this group of people. (This would be similar to how Congress granted US citizenship to Native Americans born on reservations with the Indian Citizenship Act of 1924, after the US Supreme Court ruled in Elk v. Wilkins (1884) that they were not born "subject to the jurisdiction [of the United States]".) As above, whether the Constitutional protection against involuntarily loss of citizenship applies depends on whether they were "born or naturalized in the United States, and subject to the jurisdiction thereof". To the extent that this law applies to grant citizenship at birth to people born after the law takes effect, the court clearly said they were "not subject to the jurisdiction [of the United States]" at birth, so they would not be protected against a future law that took away their citizenship involuntarily. To the extent that this law applies to grant citizenship to people who were born before the law took effect, the granting of citizenship is considered naturalization, but there are still of the questions of 1) whether they were "in the United States" when this naturalization happens (i.e. in the 50 states or DC; unincorporated territories probably don't count), and 2) whether this group of people are considered to be "subject to the jurisdiction [of the United States]" when they are naturalized, if they were not subject to its jurisdiction at birth. I have no idea how the court would rule on this second question.
