I really saw a chilling effect in r/SyrianCivilWar after the rise of ISIS. Media showing graphic violence would remain on the site for several more years -- r/WatchPeopleDie was removed only last year -- but videos considered to be "supporting" ISIS (even if only because they showcased recent advances by ISIS or allies) started being removed from Reddit, Twitter, YouTube, and other places. I think that even LiveLeak eventually started removing some of these videos. Reddit itself started banning pro-ISIS posters.
There was plenty of all-sides hand-wringing before 2016 that social media wasn't doing enough to suppress terrorist propaganda.
"After the recent spate of terrorist attacks inspired by the so-called Islamic State, lawmakers on both sides of the aisle have called for greater cooperation from social media companies like Facebook, YouTube and Twitter in combating hate propaganda."
https://www.cnbc.com/2015/12/08/how-social-media-cos-try-to-...
The social networks banned a lot of content that would be legal to publish in the United States. They went well above and beyond removing only the "imminent lawless action" speech that falls outside of First Amendment protections. And with good reason. Plenty of lawmakers were ready to make their lives miserable if they didn't take an aggressive anti-terrorist stance.
It's fun to daydream about big American social media companies removing only such speech as would be unprotected by the First Amendment. But sites like YouTube have never offered that much latitude. Even in the pre-Google days YouTube didn't allow porn -- not even perfectly legal, mainstream porn. And of course it's perfectly legal to advertise locksmith and dental services but I don't want platforms overrun with high volume advertising for those businesses either. Finally, both Republican and Democratic legislators were talking less than 5 years ago about how social media had a responsibility to curb terrorists' propaganda, regardless of the stronger protections enshrined in the First Amendment.
I don't really like where we are with social media, but I wish that the discussions we had around these issues on HN were more historically grounded instead of centering on partisan polarization around the 2016 presidential election and its aftermath.