"Misery and deprivation of people under the Maoist system"? Utter horse sh!t. The average Chinese life expectancy literally doubled during the Mao period. For the first time in history all people in China were guaranteed work, food, medicine and housing, and for the first time in history women were encouraged to stand up for their rights. You think that just because Mao didn't wave a magic wand and instantaneously turn China from one of the poorest, most exploited and most underdeveloped countries in the world into a first-world paradise bursting with consumer goods and paved with golden streets overnight, that meant Maoism was "misery and deprivation"? The Chinese people lived better during the Mao period than any time in Chinese history prior to that. Mao wasn't a god or a wizard who could just snap his fingers and instantly transform a poor country into a rich one, but considering what he had to work with, an underdeveloped, war-torn country of hundreds of millions of people largely cut off from the rest of the world, he did a pretty spectacular job with what he had.
There's a reason why Mao and communist China were a beacon of inspiration to oppressed and exploited people all over the third world at the time. Even today, though communist China has succumbed to revisionism, Mao is still an inspiration to many around the world, for example to the peasants in India and the Philippines right now who are taking up arms under the banner of Maoist revolution. And in China itself Mao is still an icon to many who believe in socialism, justice and equality and who are disgusted by the corruption, injustice and exploitation the capitalist-roaders have brought to their country, as Mao had warned they would.
Your problem is that you're looking at and judging China from the vantage point of the West, which spent centuries developing itself at the expense of the global South and East. Mao didn't make China poor; China was poor before Mao ever existed. The so-called "misery and deprivation" of the Maoist period was nothing compared to the very real misery and deprivation before it, which was why Mao and the communists came to power in the first place.
And "horrors of the Cultural Revolution", lol piss off and talk about things you know something about. The Cultural Revolution was a period of mass political participation in which the people of China took the revolution into their own hands and stood up against capitalist-minded bureaucrats in the party. Some really good things came out of it too, such as the "barefoot doctors" campaign that transformed medical care in the countryside. The narrative on the Cultural Revolution has been dominated since by those who were its targets, namely the capitalist-roaders who wanted to turn China into what it is today. In the West all you hear about the Cultural Revolution comes from these people, from right-wingers with an axe to grind and privileged intellectuals whose class interests conflicted with the revolution, elitists who thought they were better than the workers and the peasants and deserved a bigger slice of the pie. And considering what they've done to China in the decades since, it's clear that Mao and the Red Guards were not only right to fight against them but should have been a lot harder on them than they were. If you talk to ordinary workers and peasants in China, you'll get a very different picture of the Cultural Revolution. I recommend the book "The Unknown Cultural Revolution" by Han Dongping. Also "The Battle for China's Past" by Mobo Gao. These books show you China from another perspective, a perspective completely ignored in the West: the perspective of ordinary Chinese workers and farmers.
As for why this documentary was banned, truthfully I have no idea; i'm watching it now and, though I think Antonioni has a bit of an attitude problem, I don't see anything in it that strikes me as anti-Chinese or counterrevolutionary, and I say that as a communist who, obviously, sympathizes with the Chinese revolution. I think the CCP was expecting a glowing portrayal of China and instead got a more sober portrayal and didn't like it. They may also have been irked by his lack of consideration, for example when he was asked not to film sensitive locations like Mao's residence and went and did it anyway. Nevertheless I don't think the documentary should have been banned.
"The people, and the people alone, are the motive force in the making of world history."