Voice Search: From the Shield home screen, voice search doesn't show results from Plex. For example, with Kodi, a voice search from the home screen shows results from Kodi and lets you launch into the movie details directly inside Kodi.
We don't currently have Google Home/Assistant integration yet, which is why its not returning results from Plex at this time. That is, the full integrated voice search functionality from Google that NVIDIA has implemented.
Content from Plex isn't featured on the Android TV Leanback Launcher homescreen.
Just to be sure, update the Plex app. You mentioned being on version
184.108.40.2065 and the latest public release ie
6.5.3. The home screen should show media you recently watched. You can also control what appears on your home screen in the Android TV settings. Make sure Plex is selected.