FreeNAS has been undergoing and continues to undergo significant transformational changes (of particular interest here in the FreeNAS UI, and jail management) in recent releases. Here’s a chronology of change events.
FreeNAS 11.1 - The last version to focus on the legacy FreeNAS UI. The Plex plugin is installed in a Warden jail. The modern UI was introduced (and still quite experimental), but the legacy UI was the default.
FreeNAS 11.2 - This is the current release of FreeNAS. FreeNAS 11.2 presents the modern FreeNAS UI as the default, but allowed the legacy UI to run in tandem with it. Legacy plugins that were already installed could continue to run, but were no longer being updated. Consumers had the option of staying with the legacy plugins, which used a Warden jail manager, or migrating to current plugin versions on the modern FreeNAS UI, which use the new iocage jail manager.
FreeNAS 11.3 - This is the upcoming version of FreeNAS. The legacy FreeNAS UI and Warden jails are removed altogether.
So how do these changes impact on Plex support documentation?
In the support article Where is the Plex Media Server data directory located? the PMS data directory is located in ${JAIL_ROOT}/var/db/plexdata/Plex Media Server/. This is true for legacy FreeNAS (FreeNAS 11.1 and below), and for users still using Plex in a deprecated Warden jail in FreeNAS 11.2.
However, for those users who have migrated to using Plex in the newer iocage jail on FreeNAS 11.2 and for future releases of FreeNAS (FreeNAS 11.3 onwards), the PMS data directory is located in  ${JAIL_ROOT}/Plex Media Server/.
Can we please feed this information through to the Plex documentation team?