If the government suppresses the history of the event in China, how are the protesters of tomorrow supposed to know to think twice?

Actually, everyone knows about it and people talk very openly about it off the record, but no one is willing to talk about it in a recorded medium with a Westerner.

don't think people born after 90's have any idea of those, or 99% of them have no idea whatsoever, so yes, history can be revised at will, at least in the short term.

Indeed. They also didn't have the omnipresent surveillance back in 1989. Hard to imagine how the student movement back then would have gained momentum using today's communication tools (with the government backdoors and all).

Surveillance and an all-powerful regime has well and truly snookered the Chinese, and it seems their autonomy will not improve for a very long time.

Except that history isn't taught in China.

btw, those students were communists protesting the privatization of collective farms among other things under Deng.

In an authoritarian state, any spontaneous popular movement is dangerous to the leadership, even if it aligns with their nominal aims (maybe especially then). The CCP has been "Communist" in name only for a long time, but Deng Xiaopings market reforms were probably the most obvious breaking point. It's not unsurprising that students who had been taught about the inevitable victory of Communism would protest against this change of course, especially given the surrounding economic instability.

Although Communism and Democracy are frequently seen as opposites in the West, Chinese propaganda doesn't hesitate to claim Democracy for themselves (there are elections after all) and volunteering in various state-adjacent organizations is encouraged. From the point of view of an upstanding party member, organizing a student rally probably seemed like the right thing to do, in good old revolutionary tradition.