Maybe the word "censorship" is too loaded, but the claim that big tech platforms permit a narrower range of expression than they used to is so uncontroversial that it's barely worth defending. That's not always a bad thing; many of the people who've been banned in recent years are noxious pricks whom I don't miss, but what happens when the people who've been banned from Twitter and YouTube, kicked off AWS, terminated by Cloudflare and blocked by Visa and MasterCard decide to start a Substack because its commitment to free speech means it's the only place that will host them? I won't name names but I can think of, for example, a couple of semi-prominent figures who have been banned from other platforms due to their, ahem, "heterodox" views on vaccines, who now write on Substack and reportedly make a very healthy income from doing so. And worse actors might join the platform if they haven't already.
Is it really such a conspiratorial "slippery slope" to suggest that if the people who've been banned from everywhere else grow a huge, lucrative audience on Substack, then Substack is going to come under increasing amounts of pressure to kick them off - pressure far greater than what it's received so far in the face of lesser controversies? What form might this pressure take?