In broad strokes the line is simple: moderation is ensuring content stays on topic, censorship is limiting the topic. In practice it gets thorny. But we don't need to solve the concept in its full generality. This would only apply to sites that reach the size such that "public square" is a reasonable description of their social relevance.
The problem is these sites want to be the destination for communication, while also shaping that communication according to some narrow political aims. Also throw in maximizing engagement. These constraints should be seen as inherently contradictory. If you want to be the de facto public square, you should not be limiting topics among interested parties. I'll leave the specific verbiage to the lawyers, but the target is clear enough: people interested in engaging on some topic (barring illegality) should not be prevented by the site in engaging on that topic. I see nothing inherently contradictory or impossible about any of this.
