That's a strong assertion. Do you have any evidence to back that up?
AIUI, the law governing this (at least in the US -- which is relevant since we're discussing US state laws being challenged in the courts) is the Communications Decency Act of 1996[0], with section 230[1] of that law being the pointy end of the stick WRT these particulars. I would also point out that the above serves as a front-end (in that it allows the courts to reject lawsuits misdirected at platforms, when they should be directed at the source of the speech) to the First Amendment[2].
Please detail where, exactly, any of the above supports the claim that "content creators were encouraged to use and invest in building an audience on these platforms based on the tacit understanding that they would behave like content neutral common carriers."
Perhaps I'm missing something important (which is certainly possible) and I'd appreciate being enlightened. As such, I look forward to your response.
[0] https://en.wikipedia.org/wiki/Communications_Decency_Act
[1] https://www.techdirt.com/2020/06/23/hello-youve-been-referre...
[2] https://en.wikipedia.org/wiki/First_Amendment_to_the_United_...
Edit: Cleaned up prose.
Additional Edit: I'm responding to [3] below as I am apparently rate limited and wanted to make sure I was clear in what I'm trying to say as I am pretty passionate about it:
That's as may be, and I remember it fondly too.
However, it wasn't by government or the law that "content creators were encouraged to use and invest in building an audience on these platforms..."
Rather it was those platforms themselves. It would be easy for me to just say "sucker! you've been rooked!" and leave it at that.
However, I'll (attempt to) respond substantively by saying that the surveillance capitalism[4] business model was still nascent and there were many more options for interpersonal interaction back then.
It was inevitable that after the consolidation of "social" media that advertiser influence (since they're actually the customers -- not you) would be paramount to these platforms.
And that's what drives the moderation/censorship folks are complaining about -- because advertisers don't want to be associated with anything controversial, they just want you to buy their products/services and (the advertisers and the platforms as well as various middlemen) collect (ala [4]) as much information about you as they can to aid in that process.