The internet itself is the public square. There is an immense amount of publicly owned space online. Take for example:

https://www.whitehouse.gov/news/

One of thousands of places online that the white house fully controls, where the press secretary can say whatever she wants, accessible to anyone with an internet connection.

For some private individuals a twitter ban, coupled with bans from a dozen other internet hubs, might be a difficult hurdle to overcome, as starting a new site and then getting traffic to it is difficult due to network effects. But to claim that public officials have no means of having their voices heard besides twitter is patently absurd.