That's not what I would call a "first-party" library. Yes, it's written by JetBrains and under the Kotlin project umbrella, but we have to add it to our project

That's not what I would call a "first-party" library. Yes, it's written by JetBrains and under the Kotlin project umbrella, but we have to add it to our project like any other dependency. I can install a persistent collections library in any language. I know for a fact there are persistent collections libraries for Rust, JavaScript, Java, etc. But since none of those are built-in, they aren't used by the standard library APIs, and they won't be used throughout the wider ecosystem. And, I'm still not willing to call Java a "functional language" because I can install a third-party library for persistent/immutable collections and hopefully avoid the built-in collections as much as possible.