You can still stick to Java 8 codewise, it still works fine with apps that use Java 9 and up.
There is only one issue here. If the application. like mine, uses JPMS, it may suffer form errors similar to "The package java.util is accessible from more than one module: <unnamed>, java.base".
These error messages are caused by the fact that jars you currently provide are treated as unnamed modules, and when mixed with other unnamed modules from classpath can easily break the module inheritance hierarchy.
While these errors can be fixed by <exclude> in application POM, they are hard to localize the causing library.
All you need to avoid such issues is include the following line:
Automatic-Module-Name: org.apache.tinkerpop.<your-unique-chosen-module-name-for-this-jar>
to each jar's
META-INF/MANIFEST.MF
This would be an easy first step to begin modularization of TinkerPop while sticking to Java 8.