While I don't think the original claim is in any way obvious, and you may well be right overall, you're also making a huge mistake in your look at history - newspapers are not and have never been "public forums" (except to the minor extent that they occasionally published letters to the editor).
Instead, historic public forums were mostly related to in-person meetings - town halls, clubs, pubs etc. These have not been outlawed nor disappeared entirely, but they are almost entirely atrophied. Instead, public discussion has moved almost entirely online. This has created a complex situation, as online forums are almost always private property, unlike the forums of the past.
This situation is creating an unprecedented situation for free speech - as private mega corporations, not bound by the first ammendment, are now in control of a huge percentage of public communication. It seems pretty clear that Facebook or YouTube can't just be handled as publishers, nor as network operators, nor as broadcasters, nor as any other traditional form of communication. We will need to invent a new concept of how such communication should be regulated and moderated.
On the other hand, it's also true there is more public communication, and with higher reach, then probably ever before in history.