If some publisher, newspaper, or literary magazine refused to publish Walt Whitman, he could always try and find another publisher. But if Twitter bans Titania McGrath, there is no other Twitter for her to join because the next best alternative has 1/10000th the number of users.This is pretty much exactly wrong (the situation is basically the exact opposite). When there were a very finite number of publishers in the English speaking world and self-publishing barely existed, an idea that could not be published would only diffuse by word of mouth. When Henry Miller could only be published in France, his works were only available to a very small number of individuals even if many could be interested.
Anyone banned from every single media site still can create a website, publish on Tor, etc. The inability to get a million followers is far from the inability to get your ideas heard. The argument confuses an inability to make money with an inability to be hear (and feel free to argue about the right make money with incindiary ideas, that obviously has a different implication).
By creating the ability to bypass publisher, the Internet produced a situation where far more ideas could be heard than previously. That situation still exists (for good and less-good). The average person still depends on filters but those filters are voluntary and a substantial portion of people know they can bypass them if they wish. And I'd say present days are also wider than earlier filters.