Would you be opposed to similar scientific guidelines existing during the early 20th century, when the WASP American establishment tried to curb Italian, Greek, Slavic, etc immigration to the US by producing pseudo-scientific racial theories about the inferiority of said peoples?
They actually did, the immigration laws of 1922 I think it was effectively shut down immigration till 1965. You can also add Eastern European Jews that came in with Italians, Greeks, Croatians, Slavic populations (i.e. Polish) in large numbers between 1885-1920, etc. Before the US Civil War, it was the Irish Catholics and Bavarian-Southern Germans (Catholics as well) that resulted in the know-Nothing party, etc. Yes their were ethnic pseudo theories, much of it was in the case of the Italians, Polish, Croatians, anti-Catholic as well, and of course the for the Eastern European Jews, anti-Jewish sentiment. The Greeks were kind of lumped in with the other European groups as the average American Protestant back then did not know what Eastern Orthodox was, many of them today still don't, particularly the Bubba types in the rural parts of the USA.
With all that being said, the moratorium on immigration in the early 1920's, despite some of its motivation due to pseudo theories and bigotry, did in the end result in all those ethnic groups quickly assimilating into the broader US society so much so by the 1950's in the post WW2 era, those older ethnic neighborhoods, which helped all those ethnic groups maintain group identity (religion, language, food, music), etc and solidarity vs. a not so friendly broader US WASP culture, was already starting to end as those groups were moving out of their ethnic Urban neighborhoods into the suburbs and beyond.
But Europe is not the USA. The comparison to me is not relevant. Sweden for example just elected a Nationalist and Center Right coalition government that is wanting to maintain the identity of Sweden as Sweden, its culture, people, history, language, ancient Norse mythology and myths, etc, etc. Now, while I have many times here debated the Nordicist narrative of Northern Europeans being the standard by which Europeans are measured against in terms of culture, history, physical appearances, which also includes of course ancient Southern European civilizations (Greece and Rome) were originally more "Nordic", blah, blah, blah (we have DNA evidence refuting that), I 100% respect their right to ensure Sweden remains Sweden and native Swedes do not see their culture, traditions, and their own native ethnic identity ended due to illegal migration. As long as Sweden is not a threat to any other country that it borders, etc, what they do for their internal affairs regarding immigration, or lack their of, is 100% their business.
I see Giorga Meloni is potentially going to be the first female Prime Minister of Italy. Of course, the far left will call her Nationalistic, hard right, etc, etc (you never here them talk of far radical communist any longer it seems). Again, she committed to Italy's role in NATO (it was a charter member) but she is obviously no fan of the EU and was no fan of Merkel when she was running Germany (She probably does not care for Macron either). But she seems she is no longer going to allow Italy to be a entry point for the Corporate interest in Europe, the EU and the likes of George Soros to continually be the entry door to illegal immigration to provide cheap labor (which is what this is all about when you do a deep dive).