There is only one global setting for subtitles, on or off. Normally you want subtitles for movies, but for live TV that is usually in your native language so this is not needed.
Also turning subtitles off helps with the problem with transcoding, when subtitles are on the live TV is constantly transcoding which is totally broken on Plex (I have send several messages about this).
On my Sony TV the app crashes when trying to watch live TV from Plex, I cannot even disable them manually…It works with the HDHomerun app so the problem is obviously Plex…
SO PLEASE ADD AN OPTION TO DISABLE SUBTITLES FOR LIVE TV!