Yes, open source is open source.
Plex already uses the x264 encoder for the H264 transcoding.
You can find them in your “Plex Media Server\Codecs” folder.
It’s not the encoder that is the problem, it is the work that the plex developers will need to do to edit the python scripts, the device profile XML’s and the trancoding profiles for ffmpeg.
Its a question of time/priority I guess.
Of course it will also affect the people using old hardware like an old NAS, so Plex will have to stop supporting these or they will have to detect the hardware some how to be able to see if it can support HEVC, if so transcode to HEVC.
It will involve a lot of work but even just implimenting the encoder will be a step in the right direction I think, maybe a beta channel for HEVC.