The libzfs-0.5.jar version is bundled with Jenkins.war. Apparently it is built from source code which is available from both https://java.net/projects/zfs/ (branch "libzfs-0.5", SVN revision 150) and from https://github.com/kohsuke/libzfs4j (tag "libzfs-0.5" = https://github.com/kohsuke/libzfs4j/tree/libzfs-0.5, commit https://github.com/kohsuke/libzfs4j/commit/400e25e5a17993b5f3c167044d260a51d3d7dc0d).

Either way, the bundled code is from Mar 18 2009 while the head of GitHub version marched on to Sep 2013 and the java.net SVN stopped Mar 2013 at revision 180 equivalent to https://github.com/kohsuke/libzfs4j/commit/16dab80e3fee3c23511e2f958fb631cc9bc25874 with a few more fixes to the differences between OpenZFS and legacy (now again proprietary) ZFS codebase and ABIs.

As part of this improvement, the libzfs.jar codebase should be revised to accommodate both proprietary and open ZFS ABIs (see JENKINS-41932 for related work), and the modernized version should be shipped with Jenkins. Also note the story JENKINS-41949 that Jenkins should enable ZFS support on OSes other than "SunOS".