"Freedom of speech" does not simply refer to the 1st amendment - the concept has existed much longer than the USA has. I don't think GP is arguing that shadowbanned users have a "right" to use HN, instead they're saying that it is somewhat unethical because it is a form of lying.

As for shadowbanning being a key tool in managing a community - In HN's case, I imagine any shadowbanning system could be easily defeated, as a malicious actor could create a new account for every comment.