TL;DR the exercise of advertiser preferences not the shift in public discourse is the true cause.

I agree with your account in every way, but one and I recognize that I might be wrong about it.

In my opinion, it was not public discourse that shifted first. Instead, in my opinion, it was advertiser choice to not have their ads shown alongside certain content be it search results, YouTube videos, tweets, Facebook posts, or whatever else these tech companies were carrying.

I believe that ignoring advertiser preferences in this discussion is harmful in that ignores their impact on both business choices and public discourse such well-meaning commenters here end up talking past each other needlessly.

If we consider the role of advertisers in this discussion, then the comparison of these tech companies to telephone, radio, and newspapers becomes more interesting if not more apt. The advertisers don’t have any sort of explicit freedom of association given to them by the tech companies about what kinds of content they are allowed to associate with, but the point becomes moot with advertisers being customers allowed to take their business anywhere.

1. Taking the comparison of telephone, what if there was a phone service that provided free calls with the caveat that the caller or the receiver had to listen to an ad? What if that had was in some way, shape or form related to past or present telephone conversations? I would personally find this feature annoying to have some third-party delay my conversation, but I can see where this might be useful for people who would not otherwise be able to afford the service. I assume that most people who use telephones would feel similarly to myself.

2. The comparison to radio, newspapers, and television is less interesting in that both mediums are essentially curated, but both do run ads and/or sell subscriptions. Some of them even offer services that come about from gaining expertise in their domains or being in close proximity to content creators in their respective mediums (record labels, television and movie studios, recommendation sites, marketing firms, public relations experts, etc.), which in turn can be funded by subscriptions, ads, or investments.

Consequently, there are similarities between these media. First of all, these tech companies offer services that allow ordinary people to broadcast themselves to a wide audience much like radio or newspapers. Second of all, these tech companies make a majority of their incomes from a combination ads and subscriptions. Third of all, users of Facebook, Twitter, Google and the like have conversational expectations much like those of telephone conversations: even the people working in radio, newspapers, and television feel really strongly negative when any other party tries to change their message or silence them.

Like some of the other commenters here, I too feel that these tech companies have made a mistake in policing content the way they do. I believe that they did so to this degree wholly to appease advertisers. Had they been laser-focused on preserving the integrity of what their users posted before the demands their advertisers then things could have been very different.

If I was an advertiser and I only wanted to show ads to people who like cat pictures, but absolutely never show those ads to people like pictures of other animals even if there are pictures of cats in some of those pictures then these tech companies should build the tool for that.

In my opinion, these tech companies should be even more of a transparent marketplace between the ad buyers and content producers and the free association that both these groups are allowed to have with other and not police or manipulate content or that is permissible by law (keep things chronological with a user being able to keep a list of favourites). Then this whole discussion would boil down to which set of advertisers pulled sponsorship from which set of content producers and vice versa.

Yes, this is all easier said than done. Yes, the thing that it would all boil down to is potentially a problem in itself. Yes, these tech companies still have to follow laws that can be in conflict with laws elsewhere or some set of values in general. Yes, people would still take tech companies over something else related or not.

But, at least then, we wouldn’t we talking about this.