It would be good if the subtitle track could be included in the frame.io upload, either as a subtitle stream in the mp4 container, or as a sidecar file. Either way it would be useful to be able to turn them off and on.
Even better would be the ability for reviewers to make edits to the subtitles in the frame.io interface, and galaxy brain level better would be for those edits to be brought back to Premiere.