(1) You wouldn't want someone to secretly remove or demote your own commentary. But secretive content moderation is extremely common on today's major platforms. In order to be heard there, you would need to fight back against the practice, and you cannot effectively do that while keeping secrets yourself.
(2) Undisclosed content moderation does not express any kind of message, and therefore the platforms' use of it may not even be protected by the first amendment.
#2 is currently under discussion in a few cases before the Supreme Court:
Re: #2
How is it a free speech issue when someone kicks you off their property? It has nothing to do with speech so why would the first amendment be involved?
See the linked tweet for a more lawyerly argument in defense of shadow banning. The question before the court may hinge upon whether or not shadow banning expresses a message.
(I am not commenting in this message on whether an HN issue may exist or should be let go. On those matters, I am still reading.)