Enough speculation, I just did the tests.
Configuration is an R2LC-C08, generic RS power supply, 6-11207 Legacy Lionmaster RS6 board, and a TMCC motherboard. Motor drive is a standard Lionel DCDR running a can motor on my test bed.
Running in TMCC mode, a Legacy 6.0 board works fine as a TMCC board. Obviously, no quilling whistle, but everything else seems to work. I get all the sounds as I'd expect, and all the crew talk if you're into that sort of thing.
Running in TMCC mode and setting the sounds to Legacy, breaks some major things. Specifically, the whistle and bell no longer function at all. Crew talk, tower, chuff, work fine, just not whistle or bell. That doesn't seem to be a useful option. Obviously, they're expecting the 9-bit data that we spoke of. Most likely, the single hit bell and the quilling options necessitated the use of the 9-bit serial data. For the energetic, you can probably figure that out from the Legacy protocol specification, I'm not that energetic.
To wrap up the testing, I used a Legacy S02 R4LC in my test setup and repeated the tests. First off, the Legacy R4LC could not get motion at all, reprogramming, etc. no change. So, using the Legacy R4LC is a looser. However, obviously, the Legacy R4LC had no issue making the quilling whistle happen.
Bottom line? No quilling whistle for TMCC locomotives. However, as Pete says, it does open up the option to use early Legacy boards if they're available to expand your sound horizons.