TL;DR:
> Look at Reddit's ban on toxic subreddits. They found that banning toxic subreddits reduced the toxicity in the website as a whole, and that "post-ban, hate speech by the same users was reduced by as much as 80-90 percent."

No, they didn't

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> They found that banning toxic subreddits reduced the toxicity in the website as a whole, and that "post-ban, hate speech by the same users was reduced by as much as 80-90 percent."

This only holds for the creative (and frankly, dishonest) definition of hate speech that the study[0] used.

> First, we automatically extract
terms which are unique to the two subreddits that were banned due to hate speech and harassment.
The resulting term list includes a number of words that indicate hate speech, as well as some
other terms that appear to be specific to the Reddit context. We then qualitatively filter these lists,
obtaining a high precision hate lexicon. These lexicons are publicly available to the community as
a resource.

Not only did they start from a corpus of _words unique to the banned subreddits_, they "qualitatively" filtered it down, with no description of the procedure used. Anyone who's spent any time on smaller subs knows that they, like any online community, develop their own lexicons, up to and including jargon[1]. Everyone who's ever gawked at hate subs like /r/coontown or /r/shitredditsays know that this tendency is on overdrive. A reduction in the vocabulary _unique_ to the banned subreddit means that a ban refugee from /r/fatpeoplehate could be just as active on /r/HamplanetHateMail (a non-banned sub mentioned in the study) but would be counted as vastly reducing her hate speech, as long as she switched from /r/fatpeoplehate's "landwhale" to /r/hamplanethatemail's preferred "lard-ass" (I made these examples up, as I've had little exposure to the fat-activist/fat-hate corners of the Internet in particular).

What this study actually shows is: "usernames from banned subreddits don't go to other subreddit's and communicate with the same username in the banned subreddit's hyper-specific jargon". Which is to say, "when a community is broken up and dispersed among other communities, the _specific_ inside jargon of the community is a lot less common, with no reference to whether the substance of the comments have changed".

I mean, duh.

This would perhaps be excusable in 2015 or something, but one should really know better by now than to uncritically cite popular coverage of sociology studies, particularly social-justice-aligned ones, without at least skimming the paper. Studies on hate speech are particular easy to check, given that (as this study puts it), "there is no universally accepted definition of the phrase"; at a minimum, seeing what they actually define as hate speech is core to understanding the findings. In this case, the authors start with a standard definition from the European Court of Human Rights and torture out a metric that's convoluted, qualitative (in their own words!), and full of holes that the authors don't even pretend not to fill with their own biases. (Seriously, I recommend reading Section 2.2)

[0] http://comp.social.gatech.edu/papers/cscw18-chand-hate.pdf
[1] Some examples: "rentoid" or "renthog" on /r/loveforlandlords, "monkee" on /r/PoliticalCompassMemes, and famously, "shitlord" on /r/shitredditsays back in the day)