Also, what would happen if you said on social media that you thought the tiananmen square massacre was an atrocity?

Literally nothing. Discussion and much more intense criticism of the Deng Xiaoping government's handling of the June 4th incident exists on xiaohongshu.

Are you really so daft that you believe people are getting thrown in prison for internet comments? They simply delete the garbage.

Virtually every Chinese person has access to a VPN. The government does not care. Because the goal isn't total control over the information sphere, but rather to make Facebook slow and inconvenient enough that you decide to use Weibo instead. The western internet is blocked as industrial policy to advantage native Chinese companies and prevent the west from having algorithmic dominance over the information sphere of the Chinese public.

If you believe that the government is seriously concerned about Chinese people being exposed to a typical reddit anti-china circlejerk, it is you who has fallen for the propaganda. How would China be able to handle students studying abroad coming home after being enlightened?