In the May 2020 Update to Windows 10 and Xbox One, Microsoft added a CreateFromStreamWithIndex method to the TimedTextSource class that enables image-based subtitle support for media playback with the MediaElement player.
I’m sure the Xbox folks are heads down on the new client, but is it on the roadmap for Plex to add support for this API? We always have subtitles up in my household, and the constant need to hunt for text-based subtitles (since my server doesn’t have the horsepower to transcode on the CPU) is easily the single biggest headache with our current setup.