It would reduce his reach but it would not prevent him from writing these messages and publishing them, or interested people from finding them (pretty much ever

It would reduce his reach but it would not prevent him from writing these messages and publishing them, or interested people from finding them (pretty much everyone would know they can follow @donaldtrump@whitehouse.gov from one of the free speech Mastodon instances or whatever within a day), so I don't really see what the problem is. It's just that he wouldn't be promoted by Twitter.