You are correct the Captioning plugin is a DFXP sub-set solution that uses an external xml file. Adobe has added a SMPTE-TT plugin in OSMF 2.0, you can see it in the samples directory when you unzip the OSMF zip file.
1. My understanding is that both those plug-ins allow the ability to point to DXFP or SMPTE-TT files from the player side, which works fine for VOD. Is my understanding correct?
2. So what support does OSMF 2.0 have for in-stream captions in a live stream.. Does it support onTextData or onCaption data events like JWPlayer?
3. What are upcoming plans in OSMF for in-stream/live captioning?
Re 1. Correct.
Re 2. There is nothing stopping you from adding your own client handler for any in-stream callback you wish. Just set the clientHandler on the VideoElement: