I do not understand the eagerness to cede strong individual rights of expression to faceless institutions. Do you expect these institutions to be on your side? They will happily shut you up, permanently, the moment they do not like what you have to say.
As the other poster said, our right to speech is not something a corporation needs to abide by, it's something the government needs to abide by (at least in the US).
I see your point below about basically that we should have a more expansive view of the Right than the scope of its actual legal application, and, while I think it has merit, I ultimately think it's just a normative view that most of us simply don't share.
If I don't like Twitter's policies, I won't use Twitter (I already don't, and would be even less inclined if someone like Musk owned it). Not to mention it's not clear that Twitter is even censoring speech--there is plenty of garbage on Twitter. The notion it's even an example of censorship is laughable, actually.
Censored data presents a selection bias issue, but from what I can tell, Twitter's censorship seems to have little rhyme or reason. It is neither effective at suppressing lies and propaganda, nor effective at permitting reasonable discourse that falls afoul of some mob's opinion.
They still have every right to do it, but I'd be more interested in the merits of censorship if there was any institution that seemed to be doing a half-decent job of it.
I do not think that social media platforms should be obligated to publish anything any individual wants published on them, and I do not think they should be legislated into a particular attitude toward speech. But a permissive default attitude with limited restrictions seems preferable to me, and such an attitude amounts to a policy of free speech in a facially obvious sense.