My use-case for the plugin-usage-plugin is to determine which plugins can be removed without impacting jobs. As part of this, I often use it to track down which jobs must be modified before safely removing a plugin.

Job properties, such as from rebuild or throttle-concurrents, are always reported as used. I think this is because they save config in every job, whether that config is enabled or not. This is misleading, because a disabled feature can be safely removed without impacting a job.

Here is a partial example from a FreeStyleProject's config.xml:

<project>
  <properties>
    <hudson.plugins.throttleconcurrents.ThrottleJobProperty plugin="throttle-concurrents@2.0.1">
      <categories class="java.util.concurrent.CopyOnWriteArrayList"/>
      <throttleEnabled>false</throttleEnabled>
      <throttleOption>project</throttleOption>
      <limitOneJobWithMatchingParams>false</limitOneJobWithMatchingParams>
      <paramsToUseForLimit/>
      <configVersion>1</configVersion>
    </hudson.plugins.throttleconcurrents.ThrottleJobProperty>
  </properties>
</project>

I would only expect this to report as used if <throttleEnabled>true</throttleEnabled> were present.