[throwaway894345]

Whether or not Twitter the platform has some technical feature in common with the public squares of olde is missing the point. The concern is that a significant volume of our national dialogue (and indeed the dialogues of many nations) is hosted on Twitter, to the extent that many people (including the very same people who think Twitter's "censorship" is a Very Good Thing) are concerned that Twitter is a vector through which foreign state actors can and have indirectly influence democratic elections (if state actors can do it indirectly then that implies that Twitter the company can do it directly). Even if you aren't convinced that Russia used Twitter to game the 2016 POTUS election, there's a larger umbrella of concerns about the outsized influence of social media companies (for example, the various arguments levied in The Social Dilemma).