As one of those people in question, it was never unthinkable. Even during the 2000's, it was common to ban people from MSN chatrooms. Before that, it was common to ban people from IRC channels, individual IRC servers, or even entire IRC networks. Did the GNAA have a right to "be platformed" indefinitely wherever they wanted?
Also, the "censorship" in question more often deals with things like insults and other incivility, spamming, death threats, etc. Not mere "political" speech. I doubt many folks were banned from /r/SCOTUS for politely saying "I politically disagree with this ruling". We see via screenshots in the amicus the sort of stuff that was actually moderated.
Indeed, the "censorship" which spawned these laws was the banning of a dude actively calling for violent insurrection against the government, and receiving it, and continuing to encourage it during the violent insurrection. That's the "political speech" the bill authors had in mind when drafting it. It's possible that the "censorship" in question is all that stopped the putsch from succeeding. One can see why fans of said dude and his insurrection were so upset by that.