This is how the US has worked for at least 100 years:
https://en.wikipedia.org/wiki/Motion_Picture_Association_fil...
https://en.wikipedia.org/wiki/Hollywood_blacklist
https://en.wikipedia.org/wiki/Comics_Code_Authority
https://en.wikipedia.org/wiki/Parental_Advisory
From just off the top of my head
Further I want to draw a distinction between a rating group which certainly encourages censorship but does not have the ability or authority to carry it out and what we are seeing here. The Comics Code Authority worked by voluntary agreement of mainstream distributors and the underground comix scene immediately popped up to challenge it. The Hays code was enabled by a (IMO) bad supreme court decision and after that was overturned quickly crumbled to a more reasonable (again IMO) system of rating. The Parental Advisory labels, despite the intentions of their founders, were reduced to being audio nutritional fact boxes. Any traction they might have got was granted them by distributors like Walmart and collapsed shortly thereafter when the internet changed music distribution.
Amazon had to choose between appointing itself a censor or being complicit in fomenting violence via offering their services. Both choices suck, but that's sometimes the way cards lie.
> once people who are plotting bad things all move to end-to-end encrypted platforms like Signal it will be that much more difficult to figure out who the bad people are.
I think society's main problem here is the normalization of conspiratorial thinking and the radicalization that ensues. People have always been able to plot in secret and I'm not worried about law enforcement hypothetically being unable to monitor all wrong-thinkers thoughts in real time.
In some respects I think this recent spate of censorious actions (Amazon, Apple, Google, Facebook, Twitter) could be the ideal outcome in that they've always had the implicit power to censor and the public has pretty much shrugged. If society is now forced to reckon with how much of public life it has ceded to Mr. Zuckerberg's whims all the better. If decentralization comes to the web, and tech monopolies and oligopolies are cracked open to competition I will shed no tears.