Seems like the latest version (3.13.1) of Plex client for LG smart TVs was released a few weeks ago, but I can't find one for my LG projector (PF1500W) running webOS 3.00.17.
Its Content Store is still stuck with an old version (3.8.0) which has a serious issue preventing me from using it. While I almost gave up on using Plex client for webOS, I'd like to give a last shot hoping this new version might have fixed some issues under the hood.
I guess the Plex dev might need to poke LG Content Store to make updates available to this particular device. That was how XPlay got released for this device when its developer (thankfully) didn't bother to let them know it's not shown up in the store.