From the majority in Moody v. Netchoice, LLC:
The laws, from Florida and Texas, restrict the ability of social-media platforms to control whether and how third-party posts are presented to other users … [including by] requir[ing] a platform to provide an individualized explanation to a user if it removes or alters her posts….
Analyzing whether these requirements are sound, the majority held, “means asking,” as to each kind of content moderation decision, “whether the required disclosures unduly burden” the platforms’ own expression:
[R]equirements of that kind violate the First Amendment if they unduly burden expressive activity. See Zauderer v. Office of Disciplinary Counsel of Supreme Court of Ohio (1985). So our explanation of why Facebook and YouTube are engaged in expression when they make content-moderation choices in their main feeds should inform the courts’ further consideration of that issue.
For more on that “main feeds” question, and on the Court’s not deciding the First Amendment questions raised by any of the platforms’ other functions, see this post. As to the Zauderer “unduly burden expressive activity” standard, especially as applied outside the original Zauderer context of commercial advertising, see NIFLA v. Becerra (2018).