"Freedom of speech" exists separately as a philosophical concept, not as legal doctrine implemented by the First Amendment of the United States Constitution. The latter is something I have always considered a shockingly Americentric view. I have seen people go so far as to say that the First Amendment defines what free speech is — which is kind of like saying that the Second Amendment defines what a gun is.

Marsh v Alabama found that private owners might not have the ability to refuse protected speech on their publicly accessible property, so it's not so simple as just being able to declare they're a private entity and that is that.

"In Marsh v. Alabama (1946), the Supreme Court ruled that a privately owned company town could not prohibit individuals from exercising their First Amendment rights to free speech and religion"

I'm not saying bluesky is the exact same, but it seems pretty damn similar to the private sidewalk that the Gulf Shipbuilding Corporation privately owned.

That doesn't work in practice when platforms like BlueSky serve as the defacto public square.