Hi Matt,
I just this minute tried loading a midi file onto the Melody track and saving it as a BIAB file. With BIAB 2020 (730), the BIAB file saved as MGX.
I'm not absolutely certain about this but I think the file types that have been done away with are MG1, MG2, etc., which used to be songfiles based on the built-in styles.
Sooner or later, John Ford might jump in and enlighten us a bit more. He's much more versed in how BIAB has grown historically than I am.
All the best,
Noel