It's part of Java/Jakarta EE.
https://jakarta.ee/specifications/platform/10/apidocs/jakart...
https://jakarta.ee/specifications/platform/10/apidocs/jakart...
If it's part of J2EE, it's in practice "part of Java" since the JDK comes with the J2EE packages built-in...
That is, it came with the J2EE packages built-in, until Java 11 decided to break everything, and force people to get each piece of J2EE from a separate upstream project, with AFAIK no "all of J2EE" uber-jar you could simply copy into your project to restore the status quo. It's no wonder so many projects are to this day stuck on Java 8.
Are you sure about that? I just downloaded the Java 8 JDK, and javax.json is not there. And the documentation doesn't mention it either. What am I missing?