I just became aware that Mp3tag now writes select id3v2.4 frames to id3v2.3 tags.
I'd like to suggest that Mp3tag write any id3v2.4 native frame to id3v2.3 tags. There's just very little reason to do otherwise. It would also simplify the documented behavior as then id2v2.3 and id3v2.4 would behave the same.