Facebook has a limited amount of mitigation that they can do because of the nature of WhatsApp. Trying to suppress Fake News is hard when you can't even read the content
This seems like it could only be implemented client-side. It also seems like it would require rapid and frequent updates to the ever-growing blacklist. That in turn would require a way to figure out what hashes needed to be blacklisted, which implies either less-than-ideal cryptography practices or the ability to decrypt messages. Neither is desirable for user privacy.
I have a rule I use in computing: whenever you think you want a blacklist, you probably want a whitelist.
Well these fake viral messages are usually quite temporary so you could expire the cache after some time and if some messages have particularly long “legs” just re-send them to clients after their TTL expires