I'm fairly active on Reddit and I've literally never seen that happen. That's because the Chinese people so deep in historic revisionism often don't even care to begin with and can't even access Reddit in the first place, at least not "easily".
It's usually people with a "anti-China" position who bring up Tienamen square, to conflate it with whatever a given submission is about. As in: "Huweai must be spying because the Chinese government is authoritarian evil because Tienamen square".
> This is another tactic used by propagandists.
Just like conflating issues to give the impression to fight for the only "just cause" .
Redditors will gleefully jump on (by voting to the front page) anything even remotely critical of China. I suppose it fulfills some deeply-rooted sense of 'defiance' in them and gives them some sense of satisfaction. It's basically the childish game of "oh, you don't want me to touch this? I'm going to touch this over and over because of that".
The net effect is that they will judiciously post, for example, the anniversary of Tienanmen Square every year without fail, but fail to mark the dates of equally tragic, but unpatriotic or unflattering events. When was the last time the anniversary of the Mai Lai Massacre or No Gun Ri was upvoted to the front page? How about the anniversary of the invasion of Iraq?
It's ironic (and very telling of their motives) that the events they have complete freedom to discuss never receive equal attention or criticism.