First amendment doesn't mean you'll be given space anywhere and anytime you want. Your rights end where the rights of others begin. They allowed you to post things onto their property for a while - doesn't mean you have a right to that forever. You're free to set up your own website on your own hosting and/or under your own management - and thus your own rules.
But yeah shadowbanning really isn't nice and I agree that it shouldn't be used against actual humans who just wanted to say something. It's an important tool against bots and propagandists (I mean state-sponsored actors here) though.