Another update, I found this through google:
Implementing the HWE kernel now just to be sure it’s not the problem. Wasn’t needed for the standard free/non-free versions of the iHD drivers, but you article says it may be needed for some GPUs in the intel list.
This did not fix anything, but I located /var/lib/plexmediaserver/Library/Application Support/Plex Media Server and purged the contents of Drivers and Codecs. This forced a download, and it seems to have the iHD driver, but still using sw encoding or decoding in 1080p HEVC to SD h264 transcoding.
➜ Plex Media Server find . -type f -name ‘*.so*’
./Drivers/icr-1355230ec27e669c3e46f537-linux-x86_64/libigc.so.1.0.1
./Drivers/icr-1355230ec27e669c3e46f537-linux-x86_64/libigdfcl.so.1.0.1
./Drivers/icr-1355230ec27e669c3e46f537-linux-x86_64/libopencl-clang.so.14
./Drivers/icr-1355230ec27e669c3e46f537-linux-x86_64/libigdrcl.so
./Drivers/icr-1355230ec27e669c3e46f537-linux-x86_64/libocloc.so
./Drivers/icr-1355230ec27e669c3e46f537-linux-x86_64/libiga64.so.1.0.1
./Drivers/imd-9e4cd70adad80b4a3ed849b3-linux-x86_64/dri/iHD_drv_video.so
./Codecs/46f74ab-560174306fe167a5978a79dd-linux-x86_64/liblibx264_encoder.so
./Codecs/46f74ab-560174306fe167a5978a79dd-linux-x86_64/libhevc_decoder.so
./Codecs/46f74ab-560174306fe167a5978a79dd-linux-x86_64/libaac_lc_decoder.so
