For clarification: The 1st amendment ensures that the government may not stop you from saying what you want. This is a separate issue from private companies removing/banning you. Each company has their own right and freedom to be as dickish as they want with regard to deleting your account etc - but that is not related to the 1st.
That's not the only example. There's extensive case law that establishes how both governments and businesses have to either allow or curtail speech under certain conditions. A business can't legally compel you to do say something or wear a piece of clothing in a way that discriminates against you. A sex shop can't just open anywhere because, while it's been argued that they should be allowed as a form of free expression, they tend not to be considered as such under the spirit of the law. A person can be held responsible for the aftermath of shouting "fire" in a crowded room, even though this speech is superficially supported by the first amendment. Point being, we make decisions about freedom of speech that don't necessarily follow the letter of the constitution or stay within the bounds of the government.
Freedom of speech isn't just a law. It's a social principle upon which America was founded. If all meaningful communication is dominated by too-big-to-fail businesses with AI that can scour all correspondence, a reality we are rapidly approaching, then freedom of speech becomes meaningless. The situation becomes worse when these companies are all politically aligned with the regime. This is why we can't ignore how private companies regulate communication through their systems.