https://techcrunch.com/2020/12/22/substack-explains-its-hands-off-approach-to-content-moderation/

Content moderation has been a thorny topic in 2020. And when I say “thorny,” I mean in the sense of having multiple congressional hearings on the subject. Twitter and Facebook in particular have been mired in concerns around the subject, fielding complaints that they both haven’t done enough to weed out problematic content and suggestions that they’re a censorship-happy, shadow-banning enemy of the First Amendment.

The latter appears to be the sole reason for the existence of the right wing-focused Twitter competitor, Parler.