As a Rust user, I have to say I think this is the right choice. Keep things out until they are practically dead/done. Rust covers a very large number of language use cases from C++ to Python. C++ doesn't have a std lib JSON parser either. And serde is fantastic!

Generally rust prefers to keep things out of the standard library because it's hard to change/improve a standard library. They try to give enough useful stuff and then leave all the other stuff to easily downloadable packages.

More importantly, it means that version upgrades are decoupled.

They don't have to download and install version 1 of an API for backwards compatibility if you're on version 2, and you don't have to download a new compiler version to upgrade a package as long as the package's maintainer is still willing to support the version you're on.

(That latter one being the same as with support for new revisions of C or C++ in in GCC.)