Availability of Shenandoah 2.0 in JDK

Oracle famously does not ship Shenandoah in either their proprietary Oracle JDK downstream, or their OpenJDK builds. For that matter, Oracle does not list any Shenandoah-related release notes for their builds -- that is the link you have. You want JDK 13 from any other OpenJDK vendor and look into their release notes. Automatically-generated release notes for the vanilla OpenJDK 13u can be seen here, generated by myself -- there are the whole lot of RNs for Shenandoah in hotspot/gc section.

"Shenandoah 2.0" indeed ships starting from JDK 13, and it was also backported to Red Hat 11u and 8u downstreams, at 11.0.5+ and 8u242+, respectively. Recently, the proposal to contribute it to 11u upstream was made.

See more details in the relevant section of Shenandoah wiki.

Oracle does not support it, try downloading Oracle JDK and enabling it : -XX:+UseShenandoahGC, it will not work; it will though, if you get AdoptOpenJDK.