We are working with a very high priority on JDK 11 support. We should be able to ship it in a few months from now.
> So is graal polyglot already compatible with newer JDKs and it's just native image generation that's blocking the upgrade?
Not just. We also need to upgrade the JVMCI version we use in order to support libgraal (Graal compiler as native-image in HotSpot). Also, modules make everything a little bit more complicated as well.
Native image is experimental, and I believe is a separate download now. Maybe someone from the Graal team can give more precise information about what exactly works in stock OpenJDK, but you should try that demo with Java 12 (or even 13). While you're at it, also give ZGC a try (see https://news.ycombinator.com/item?id=19886856 and http://www.programmersought.com/article/8311115057/).
We are working with a very high priority on JDK 11 support. We should be able to ship it in a few months from now.
> So is graal polyglot already compatible with newer JDKs and it's just native image generation that's blocking the upgrade?
Not just. We also need to upgrade the JVMCI version we use in order to support libgraal (Graal compiler as native-image in HotSpot). Also, modules make everything a little bit more complicated as well.