Even when you look at Nazis you will see a lack of historical consistency : shaming Nazis was impossible in polite company until something like 1941. It was possible when the Nazis were only in a few cities and not yet even a German movement, but by the time they had grown internationally to become an important part of international socialism, they were unassailable. Granted the exact point where it became possible varies somewhat depending on where you were, but not that much. Most socialists accepted Nazi control of France, for instance, as a prelude to a reorganization into a socialist state of that country. That is a viewpoint you find defended in New York newspapers in 1941 ... and there ideas were just the same back then. Kristallnacht had happened and was widely known and reported on and reports of state "disappearances" had leaked, some were well known. But that didn't matter. What mattered, it seems, is the opening of the second front against the Soviets. It is very clear that that was were the Nazis lost their victim status (I'm not kidding, they always played the victim card. I'm even willing to say that in some cases they rightly did so. Certainly a few of their grievances were legitimate)
Likewise communism and even to a lesser extent the Soviet system was unassailable in Western press until it was very clear (mostly through the Berlin wall) that the Soviet bloc was crumbling. It was still very powerful when criticism started, but it was clearly on the way down at that point. And criticism grew stronger the weaker the Soviets became. Coincidence ? I think no.
So I submit there is one line that I truly feel is a line in who we shame. It's the only line that I've consistently seen applied :
We shame the weak.
Those that either can't, or to a lesser extent those that for some reason won't, defend themselves against the shaming.
If there is a serious risk of getting attacked in the street, we don't shame ideas. That was true of Nazi ideas in the runup and early phases of WWII, and is equally true today.
We can name plenty of ideologies that have extremely unacceptable ideas and components. Ideologies that have historically or even contemporarily committed genocide and may even deny doing so (a certain EuroAsian state comes to mind), and yet ... no shaming. Religions and ideologies with laws that are racist, sexist, and frankly disgusting are a dime a dozen, and all the big ones of course have this. And let's just shut up about the role models of ideologies. Jesus may not have killed and even prevented others to lift a sword on his behalf, but to say that this is uncommon behavior is ridiculous. There is no one else who's done so. Most religious leaders demanded the opposite: that their followers fought, commit genocide and died on their behalf. And yet very little criticism, and even that little bit is getting attacked.
So, reality, sadly, is very simple: we shame, and otherwise victimize, the weak. This is the very same impulse that causes racists to attack others.