It's not a First Amendment issue, and it's not state censorship, but it is censorship.

Edit: This quote though seems to be wrong: What competency do tech giants have to arbitrate over science and health policy? … How did they get into a position of some sort of philosopher-king to be able to sit in judgement as overlords of our discourse So yeah, clearly each platform has control over what is acceptable to say on the platform. Encouraging free speech is important, but that takes work. Rule with an iron fist is the default for any platform, even blog comments.

Defining the suppression of discussion on COVID as acts of censorship has nothing to do with the private nature of the platforms. It is legal, but it is still censorship.