HotSpot JDK9 source code contains several unused VM Options which can be removed or made obsolete. All options are defined in "hotspot/src/share/vm/runtime/globals.hpp" file.

Here a list of options with comments:

1) AdaptiveSizePausePolicy - product int flag, description: "Policy for changing generation size for pause goals"

Unused product GC flag. Can be made obsolete.

2) AdjustConcurrency - product boolean flag, description: "Call thr_setconcurrency at thread creation time to avoid LWP starvation on MP systems (for Solaris Only)"

Become unused as part of JDK-8038473 "Remove support for old T1 libthread", Can be removed, because used for threading support on Solaris 8 which we have not supported for a long time.

3) ParallelGCRetainPLAB - diagnostic boolean flag, description: "Retain parallel allocation buffers across scavenges; it is disabled because this currently conflicts with parallel card scanning under certain conditions."

Become unused as part of JDK-8065972 "Remove support for ParNew+SerialOld and DefNew+CMS". Code enabled by this flag will be removed by JDK-8073466 "Remove buffer retaining functionality and clean up in ParGCAllocBuffer". Flag can be made obsolete.

4) PrintJVMWarnings - development boolean flag, description: "Print warnings for unimplemented JVM functions"

Become unused as part of JDK-8057777 "Cleanup of old and unused VM interfaces". Can be removed, because it is a development flag.