Give 'em Hell, Devils
It's not a Plex thing. It's simply a standard that video players (including Plex and VLC) have a setting to turn subtitles/CC on or off, and if the .srt file is properly embedded in the video file, then the player has the ability to turn the "words" on and off. As I said, I have no experience trying to play such files on a TiVo, whether through Plex or otherwise. I'm simply stating that you can embed the CC or subtitle information into the video file and you can turn it on or off if you use the proper player app/software. If TiVo doesn't have the proper software, that doesn't mean the CC/subtitle info isn't accessible when the content is "acquired" through other means.Subtitles are not and never have been supported by TiVo and I wouldn't expect them to be. Both broadcast TV and cable use CC, not subtitles. Remember what a TiVo's primary function is.
Subtitles are on separate tracks while CC are embedded in the video track. When you transfer a recording to a TiVo, those tracks aren't even included.
And it is "a Plex thing" as far as viewing subtitles through a TiVo is concerned. The fact that other players also support subtitles is irrelevant.