> I do believe public vs. private is relevant in terms of legal responsibility
The kind of “public vs. private” by which Twitter was once public is relevant in terms of whether or not they are subject to, say, certain SEC regulations, which are a form of “legal responsibility”, but it is not relevant to whether or not they are legally considered to be federal or state government entities under the 1st and 14th Amendments, such that they would be restricted from the kind of content-based restrictions the government is prohibited from imposing.
Citizen’s United is irrelevant. This isn't an issue of whether or not Twitter is a person protected by the 14th Amendment, but whether it is the federal or state government, and thuse restricted by the 1st and/or 14th amendments. And–as I would think would be obvious–Twitter is neither the federal government nor a state government. (And, if you wanted to make the argument that it was an agency of state government because corporations are created by states through law, that would be more interesting, but then the "private company" vs. "public company" issue would still be moot, because that would apply to all corporations regardless of whether their stock was publicly traded.)
> I can’t wrap my head around this notion that an agent of $40B public company constitutes “private censorship” do some research.
I have an undergraduate degree and some professional study (abandoned midway for an IT career) in this field. What you are doing is called “equivocation”; making an argument that attaches the import of one meaning of a word to a circumstance in which a different meaning applies. A “public company” in the sense that Twitter used to be one is a short way of saying “A private corporation with stock traded on a public stock exchange”.
It is not the same thing as a government entity subject to the 1st (federal) or 14th (state, and consequently subdivisions thereof) amendments to the US Constitution.
The kind of “public vs. private” by which Twitter was once public is relevant in terms of whether or not they are subject to, say, certain SEC regulations, which are a form of “legal responsibility”, but it is not relevant to whether or not they are legally considered to be federal or state government entities under the 1st and 14th Amendments, such that they would be restricted from the kind of content-based restrictions the government is prohibited from imposing.