From what I can tell this is a second order effect. No advisory was made against serde-yaml, it was made against rust-yaml. The maintainer of serde-yaml I believe just took the opportunity to mark it as deprecated to ensure people don't migrate there.
Yeah, I'm really disappointed reading the comments here. This is his project, he can do whatever he wants with it, including delete and forget about it. The author doesn't owe you anything. Just use a fork or another implementation. This isn't the end of the world.
I don't think anyone is claiming he is legally required to do it. Just that it would be nice. But in this case I think he did try to look for someone anyway.
