The German public were responsible for what they knew of and had control of, just like the Soviet public. These were authoritarian regimes.

Nobody is responsible for what they don't know about and are unable to learn about. In what world should they be?

Well, there are cases where you are legally responsible, like sex with minors and money laundering, because otherwise noone would ever face punishment

Whether this applies here depends on what you mean by 'responsible' in thos context, is it morally responsible?

I'm a big fan of the enlightenment principle that human beings are responsible for the things which they can control. As opposed to the belief that we are all extensions of our collective tribe, and are therefore collectively responsible for our tribe's crimes, as well as those of our ancestors.

I am not morally responsible for actions I don't commit, or actions that others commit that I had no way of stopping.

This belief appears to go against a current cultural zeitgeist of regressing to tribalism and collective/ancestral guilt. The labelling is different, but the implications are the same.

So are you responsible for things you could have known, but deliberately chose not to look into (or not believe)?