The issue isn't Donald Trump being silenced, the issue is wannabe tinpot dictators like him trying to coerce private business into doing his bidding. That's the actual free speech issue if there is one. Nobody should be compelled to host anyone else's speech.
Social media sites are not 'de facto public squares' in any way. It's trivial technically to create an alternative site to the incumbents, that's why there's already like half a dozen alternatives spawning around Trump's political ecosystem. It's just that the content on those sites is so godawful within days of them going online that everyone runs for the hills who has a few brain-cells left.
It is so ridiculous to paint people like Trump, literal oligarchs, as victims of private business lol.