Nazis considered Slavs as non-Aryans, but did Nazi linguists classified the Slavic languages as "non-Indo-European"?
What was the Nazi theory about historical linguistics?
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The Nazi theory, as summarized here, is not about language relations, it is a claim about genetics, and apparently Hitler was inspired by theories of Hans Günther. This book studies Nazi linguistics, and chapter 2 may be useful in tracking what was said in Nazi-dominated linguistics. I think that the racial theories are primary and linguistics follows that, which explains why Finns are classified as racial Aryans, and Slavs and Romani are in the "subhuman" class. That is, you can be a "racial outsider" relative to the language that you speak.
Nazism is primarily a political ideology. So it's a bit difficult to speak of "Nazi linguists" and historical linguistics is more an alibi than a theory, from a Nazi point of view. You can find more information here: https://en.wikipedia.org/wiki/Gustaf_Kossinna