“Twitter has endeavored to be an open platform for free speech — one in which the community itself does the policing.”
Twitter may indeed have more in common with Reddit than with Facebook or Snapchat, but (to judge from limited experience) there’s also a big difference. Reddit looks like a large set of loosely defined communities; anyone who reads, posts to, or comments in the /r/science subreddit (random example) is in a sense a member of its community, and this may encourage a willingness to follow the rules of reddiquette and to police those who break them. But I don’t know how the policing works (I wouldn’t have minded seeing a brief discussion), and in any case, it seems to me that on Twitter there is no community, or else the entire user base is the community—in other words, no one exactly feels they belong to Twitter as they may feel they belong to /r/science. What’s more, I think, at a guess, that much of our offline life takes place in groups and communities, some more temporary than others (the crowd at a town hall gathering or a Meetup or a comedy club may be short-lived but is nonetheless a kind of community while it lasts), and this helps explain why intemperate voices tend to be hushed up or shouted down in real life: everyone in a group feels a certain sense of belonging, and everyone knows it when the shared experience is interrupted. On Twitter, though, no one who follows Leslie Jones knows, in the normal course of things, what she’s hearing except when she says something about it, and on Twitter there’s no way to sh someone or shout them down anyway—as you say, the feature isn’t there.
It might help, as Robert Sharp suggests, for Twitter to open up its API again, or at least open the doors to developer ideas. And I wonder what would happen if someone at Twitter talked to a few social scientists. Probably John Brockman, who runs the Edge website, could suggest some.