It has been my impression that Yaesu isn't exactly forthcoming about the programming details of their radios, so it isn't especially surprising that the digital modes haven't been reverse-engineered.
This is true, but also true of 99% of all the radios supported by CHIRP. They're almost all exclusively reverse-engineered the hard way.
It's an immense amount of effort every...single...time we add a radio. Not sure if that's a well-understood detail of this process, but the volunteers that are adding support for radios for you - they're staring at walls of binary for *hours* and *days* to suss out what each bit does. We don't (in almost all cases) get docs telling us what the bits do. We have to shake real hard and then pay close attention to see what falls out :)
The volunteers that do this are often working with borrowed radios, or do it with a radio they then pass on to someone and buy something else. This is why it's hard to go back and add something like DN mode later on a driver for someone like me who doesn't have a radio to test with.
I've thought about doing a conference talk about what is involved in adding a new radio to chirp so people can see how much effort it is. I guess maybe these days it should be a youtube video :)
--Dan