He's legally free to do this, but morally is a whole other thing.
Its especially worthwhile because Musk sold himself as an absolutist of a view of free speech in which platforms like Twitter were not exercisers of free speech but actors whose decision to shape and bias content violated their users rights to free speech.
That said, I'm no expert and was just using it as an example.
Even though you can argue the free speech angle regarding news publications such as the new York times, I'm not so sure if blacklisting links to competitors such as Threads is something that sits well with antitrust agencies.