> This is the canonical meme of free speech
That's a pretty dismissive way to say "jurisprudence".
> If every arena where people actually meet is privately owned, and all of these places agree on what ideas are valid topics of discussion, the effect on society will largely be the same as if a state actor suppressed free speech.
The concept that you're edging up to here is called "State Action". It's the idea that there are times and places where a private actor meets some sort of condition that makes them act so much like a government, that we hold them to the standards that we hold other governments, including all of the amendments and their existing jurisprudence.
While the Supreme Court has held up the concept of State Action, they've narrowed it significantly. The major case where State Action was upheld, Marsh vs. Alabama, the private actor in this case built a literal town. Since then the Supreme Court has been narrowing the precedent in this case, a process called "Limiting a case to its facts". Recent cases about State Action have had the court restrict State Action to "powers traditionally exclusive to the state", which does not include things like Twitter.
> People who repeat what you just wrote usually seem to ignore it:
This is both unnecessarily insulting, it's also wrong. People do talk about this stuff a lot, myself included. The fact that your reading list doesn't overlap with anyone who knows the jurisprudence here is your fault, not my fault.
> As a current example, consider if Facebook, Twitter and reddit all agree that any discussion asking critial questions regarding transgender issues are grounds for expulsion
This might be a problem under anti-trust issues, but it's not a 1st amendment issue.
> We're not quite there yet. But there are numerous incidents that make me worried that this trend will leave us with a society that's net worse off. Or moves towards violent conflict as the suppressing the free exchange of ideas has economic consequences where those with social access to free speech arenas get better opportunities, leaving the rest without even the possibility of engaging.
This is always the heart of the "free speech on Twitter" argument, and it is always extremely poorly thought out. Are you suggesting that Twitter cannot moderate beyond the (very wide) bounds of the first amendment? Because those "Twitter mobs" you fear are protected speech; if Twitter is held to the first amendment then they cannot be removed. Or are you suggesting that Twitter do something about the "Twitter mobs", which implies moderating some people for what they say on Twitter, which isn't exactly the "free speech" you're advocating for.
You can't have it both ways.