Not sure how iOS relates to Xbone. Their are no subtitle support over DLNA on xbox, so there are no compatible formats to which to convert. Suggest anyone that wants subtile support on Xbone start here: https://xbox.uservoice.com/forums/251646-apps/suggestions/6345301-support-subtitles-in-xbox-one-media-player
Well, what I mean is the devs have said they are working on a subtitle solution for the Xbox app and the new feature on iOS looks like a big step forward in that area, so I just put two and two together and thought maybe this could be ported to the other platforms (depending on how it works of course). I'd assume they're also talking to Microsoft about any support they might need from their end to make it happen.
The devs also mentioned their main goal is for the app to match the features of other Plex apps before working on anything else I believe, so this should be on the list.