There is no ISIS subreddit because ISIS has now moved its threatening planning underground into encrypted channels where the good people at the NSA can no longer quietly observe and monitor from afar.

Arguments about whether deplatforming has merit should be automatically deplatformed via machine-learning in order to teach everyone a valuable lesson in civics. At least, if I were to create a Hacker News or a Reddit or a Facebook or a Twitter, that would be one of the first things I would build.