Currently the Old Player is the only way to playback HEVC content on the AppleTV HD and older iPad devices with Plex. The MPV player and the new Experimental player have issues with HEVC content on those devices (though third party players such as Infuse do not have the same issue, and HEVC is officially supported by Apple on those older devices).
EDIT: A possible solution may be black listing those older devices either in the plex client or server and forcing all HEVC media to transcode to h264.
Thanks
