If you want an example of something that is almost quite literally the public square, you could look at the court case robin vs pruneyard.

In this case, the government forced privately owned malls to allow political protests. So yes the government has forced public squares to be open to the public and prevents private companies from censoring political speech in those public squares.

> or a mass communication system though

Common carriers quite literally are mass communication systems. What do you think common carriers "carry"? The answer is that many of them carry communication.

Things like the phone system are for mass communication.

So yes, we have laws that effect both the public square and mass communication systems.

ISPs are also communication systems and laws called "net neutrality" laws force them to do certain things.

In fact, this article itself is about how the ISP is acting illegally and is breaking existing laws about what the ISPs can censor.