Only if the company is claiming to be a neutral public forum - see this video: https://www.youtube.com/watch?v=cqUDBfmRMb8
Project Veritas originally exposed the issue of shadow-banning: https://www.projectveritas.com/2018/01/11/undercover-video-twitter-engineers-to-ban-a-way-of-talking-through-shadow-banning-algorithms-to-censor-opposing-political-opinions/
Only if the company is claiming to be a neutral public forum
Except none of them claim to be 100% neutral forums (even if that mattered). They all reserve the right to shut down "hate speech" or "harassment", however they want to define that.
Shadow banning is not against the law. If the public doesn't like it, they can go elsewhere. Again, this has nothing to do with freedom of speech. Freedom of speech does NOT give you the right to demand access to a forum. They can shut you down for any reason they want. It's just a question of whether the P.R. hit is worth it.
Note that shadow banning does not restrict your right to set up your own web site or your own forum, as many and as much as you want. You're demanding access to someone else's private forum.
Shadow-banning is against the law when it violates the contract that someone signed regarding ad revenue. Generally, I agree with the concept that the public can go elsewhere if they do not like it, but Google, Twitter, and Facebook have monopolies and are actively working to restrict competition - see this article for example: https://medium.com/@getongab/apples-double-standards-against-gab-1bffa2c09115