In Libreelec I am using Jellyfin for Kodi. I have enabled force transcoding. I would like the subtitles in Jellyfin to be burned into the video with no exceptions. There’s an option to disable subtitle extraction on the fly but this doesn’t burn in the subtitles
Forcing subtitle burn in would require every item to be transcoded.
In Jellyfin you need to go to User Settings > Subtitles > Burn > All.
And probably change subtitle mode too to always have them on.
Then in Jellyfin add-on for Kodi you need to make sure you go via the jellyfin server (add-on mode I think?) not native mode.You can use a tool like Handbrake or ffmpeg to burn titles directly into your videos, but you’d have to re-encode the files. Perhaps not what you actually have in mind?