Can you give a non-hypothetical example of something that is a public platform?

It is called 'the Internet' and can be used to disseminate whatever you want, within or (to a limited extent) outside of the bounds of the law of the land. As long as you remain within those bounds it is generally possible to say what you want although you might have to erect your own platform - in the form of a website, netcast, 'radio' station or video streaming service like Peertube - to be able to speak freely.

Centralised corporate platforms like Youtube, Twitter and Facebook are not public platforms and should not be treated as such. The 'net is the platform and should be treated as such. Just like the old telcos were not allowed to censor the phone lines it should not be allowed for internet access providers and backbone providers to refuse access or transit because they happen to dislike the message. There is a discussion to be had on whether service providers like AWS should fall under this regime, personally I see more in a decentralised approach but there are arguments to be made for including them.

> it should not be allowed for internet access providers and backbone providers to refuse access or transit because they happen to dislike the message.

The key word being "should". If my ISP refuses me access because they dislike my message today, do I have any legal recourse?

If that ISP wants to remain protected as a common carrier you do but it will take a number of such cases to find out whether this holds. This is why I used the term should instead of is - a precedent needs to be set.

On the internet? Not really - every site is privately owned and controlled by some entity with the right to decide what does and doesn't get published on their servers. Even laissez-faire "free speech maximalist" platforms like Gab and Parler are in theory entirely private and centralized, they just choose to allow public access (under their terms of service) and lax moderation. But in practice that decision is entirely arbitrary.

WWW. There could be wide-access public radio and tv, but in practice it isn't used much.

That's what shopping centers in California said ....[1]

[1] https://www.jonesday.com/en/insights/2008/08/union-access-to...

Barring the repeal of Section 230 and some law declaring all social media platforms to be public property, it's true regardless of what the state of California says about shopping centers.