If 20% of your potential audience can be taken away by the flip of a switch (or worse, an automated algorithm with no real oversight), then it really doesn't matter who's doing the taking away. The harm is the same.
Also, if you've ever had the misfortune of helping run a forum, you would know that this isn't really workable. Without proactive moderation that goes above and beyond the bare legal minimum, a tiny minority of individuals will just take over the space and completely ruin it for everyone.
There has to be some moderation beyond that the government does to public spaces. At the same time, I don't want people to be functionally excluded from society purely for their political opinions not least of which because I believe people can change. There has to be an equilibrium, and I think that purposefully harassing minorities should be outside of that equilibrium.
I've seen this happen, but forums never had the universal user base that modern social media companies do. If you get banned from a forum, you get cut off from maybe 500 members of your target audience. If the same happens with a social media platform, you lose access to hundreds of thousands or even millions of members. The impact of the ban is disproportionate, except in the cases of the most extreme speech (like calling for literal heads to roll, for instance).
Is it a private company? How much control does the CCP need to have over it before it's recognized as an arm of the state.