the library would most likely remain OSS even after the take over
What makes you think that?
Yeah, if they don’t add malware. I doubt redis will add malware to specifically their rust client.
OSS authors can add malware too.
The risk here is they intentionally make it incompatible with Valkey, similar to how Elasticsearch client libraries complain when a newly added slogan header is missing
That's almost guaranteed. Valkey is going to have to fork and maintain their own client libraries.
I don’t think they could do that in a way that would prevent a fork from undoing it.