Serde does not require nightly, though it is nicer to use on nightly. That's the "will be stable as of the next stable release for Rust" I alluded to above.

Rocket just came out; I think it's an extremely interesting library, but https://crates.io/crates/rocket shows that it's been downloaded 618 times. It hasn't exactly taken the world by storm yet. I think it shows great potential though! But it's not a good case of showing that the Rust ecosystem largely depends on nightly.

The article you refer to contained a number of factual errors.