

No, I would not want to join such an instance but I wouldn’t mind its existence. Nobody could really federate with it. So you create a niche server in an already niche environment.
I am not convinced the conclusion “if the government runs it, the first amendment has to apply” is apt. Even if the server was run from under the house majority leader’s desk - which I don’t think it would, this smells more like an outsourced undertaking - moderation on the platform is not “making a law.” And proprietors of platforms are legally compelled to moderate in certain cases, e.g. when illegal stuff like child sexual abuse is involved.
Neither of us are legal scholars, are we. If I pretended to be one, I would say the government acting as a user on somebody else’s platform or the government running its own platform are different enough circumstances not to derive comparisons from.