Privately-run communication channels have always been arbiters of truth. Fanzines and newsletters chose what to publish. The town supermarket chose what flyers it would allow on its bulletin board. The local book store chose what books and magazines it would sell. Social clubs imposed standards on their membership. And this has always been (at least arguably) both their right and their duty.
The problem is not that Facebook does this. Unless and until they seek some form of common carrier status, they continue to retain the same curatorial rights as pretty much any other private organization that isn't a telecom. The problem is that the town bookstore is gone, the town grocery store no longer has a bulletin board, the Masonic temple and the Elks club have been converted to condos, and we have generally allowed the available media for disseminating information to consolidate into a worldwide oligarchy, with Facebook as its most powerful member. The problem is that my mom gets almost all her information about the outside world, including what's happening in her local community, from Facebook, and I honestly don't know where else she can go.
And that's a problem even if Facebook doesn't try to actively arbitrate truth, because you'd still have all the other problems: The tying of access to information to an abusive monetization policy. The tying of most opportunities for community engagement to an abusive monetization policy. The single, easy target for astroturfing and political manipulation. The way that lies and misinformation generally have to be left to spread like wildfire because you can't do anything about it without people complaining about free speech.
The internet allows a wealth of information, and you should not trust FB as the arbiter of information anymore than you should absolutely-without-question trust Washington Post, Buzzfeed News or the freaking Daily Stormer.
FB 'censoring' some ridiculously unscientific and dangerous information, like COVID19 disinformation, doesn't completely hide that. You can still find it out there, but you have to look for it.
I would agree it opens up a door. You censor one thing, you think it might be worth censoring another thing and so on. But 'slippery slope' is a logical fallacy for a reason, and this disinformation is an inherently difficult problem, we can't just look to easy answers like "don't allow censorship."
(Does anyone have a definitive "masks will not help" statement from the WHO? Rather than a "masks should not be worn" one)