Various lawsuits over the years have conferred on big tech an unusual status: a public good, held in private hands. In 2018 the Southern District of New York ruled that the right to reply to Trump’s tweets is protected ‘under the “public forum” doctrines set forth by the Supreme Court’. So it was wrong for the President to ‘block’ people — i.e. stop them reading his tweets — because they were critical of him. Censoring Twitter users ‘because of their expressed political views’ represents ‘viewpoint discrimination [that] violates the First Amendment’.
In Packingham vs North Carolina (2017), Justice Anthony Kennedy likened internet platforms to ‘the modern public square’, arguing that it was therefore unconstitutional to prevent sex offenders from accessing, and expressing opinions on, social network platforms. ‘While in the past there may have been difficulty in identifying the most important places (in a spatial sense) for the exchange of views,’ Justice Kennedy wrote, ‘today the answer is clear. It is cyberspace —the “vast democratic forums of the internet” in general … and social media in particular.’
In other words, as President of the United States, Trump could not block Twitter users from seeing his tweets, but Twitter is apparently within its rights to delete the President’s account altogether. Sex offenders have a right of access to online social networks; but the President does not.