I'm suspicious of the evasive vagueness too, and therefore suspect the OP is not talking about what I'm thinking of (though no idea what). Nevertheless:

I do think a lot of common cultural how-to knowledge was destroyed around the middle of the 20th century by the new corporate "mad men" advertising world. For example, complex knowledge of cooking was universal, passed from generation to generation from an early age, then along came heavily advertised processed foods and within a generation, vast swaths of the population have much less know-how being received from their parents, just favorite brands and how to boil the mac and cheese. So much cultural traditional knowledge was lost when it was replaced with mass manufactured "junk culture" and only later are their descendants slowly reaccumulating it.

What is a common knowledge you believe disappeared?

Given the evasive nature of the assertion, I'll eat my hat if it's not some flavor of racism, sexism, or eugenics.

Some good things German people actually did. (The ones that opposed the Nazi party), and now the _common_ belief is that _all_ Germans were Nazis

I do not believe this is true in any meaningful sense. Schindler's list, Operation Valkyrie and the like are firmly embedded in popular culture. And of course the claim that all of any nation share an ideology is absurd to anyone who thinks about it for two seconds, but I'll give you a pass on that because I don't believe you're talking about people who think for two seconds (I claim that that is also an important error in your worldview, but that's tangential).

Not here (different EU countries); we are taught in school about both sides. There are many books based on reports or letters by soldiers or concentration camp inmates describing Germans not believing in the war effort and more so not believing in the holocaust and helping people. Schindler is a famous example.

Not for everyone!