That one will never happen, because you could write a custom sort function that sorts raid units by health deficit, which is exactly what they want to expressly prohibit. The best you might get would be a selection of pre-defined (static) sort orders. By class, by level, by group, that's about all you could come up with.
Remember it's perfectly OK to re-arrange unit frames out of combat. So, you can create whatever kind of fancy sorting technique you want and perform the sort before combat starts - it just won't work when in combat. With the right coding, you can even set up a finite set of such sorts and then switch between them with a button press or keybind once combat begins... but again they all would need to be
set up while out of combat. Pays to be prepared I guess.
