When Oracle turned over the Java EE projects to the open source community, the first release of Jakarta EE was basically just a re-release of Java EE 8 (which included JAX-RS 2.1) - it used the same package space as Oracle had previously used (javax.ws.rs.*).
However Oracle required that the open source community use a different package space if they wanted to change any APIs. So Jakarta EE 9 is all about changing the package space from javax.* to jakarta.*. Oracle also required that certain acronyms be changed - like JAX-RS is now known as RESTful Web Services (but since that's a mouthful, I won't hold it against you if you want to keep calling it "JAX-RS". :-) ). So for EE9 / RESTful WS 3.0, the API packages will change to be jakarta.ws.rs.*.
I don't think that development of a "native" 3.0 implementation has started yet (other than Jersey), but what we are doing in Open Liberty betas is taking the latest RESTEasy modules and "transforming" them so that they will use the new namespace. We're using a tool called Eclipse Transformer to do that (more info here: https://github.com/eclipse/transformer
Hope this helps,