Debian's general policy is to only have one version of a software package in a release. So, absent exceptional circumstances, there would only be one version of OpenJDK. Buster has 11.
Details can be found in an [email to debian-java](https://lists.debian.org/debian-java/2019/04/msg00036.html), but in short:
- OpenJDK 8 security support (already from third parties, e.g., Red Hat) ends before Buster's EOL. So sticking with