Since kill-whole-line kills both backward and forward from point, it
seems we should expect that the first part is prepended to previous
kill, whereas the second part is appended. Which is what the command
already does.
WDYT?
Since the current behavior is explicitly documented in the code, I suppose that settles it. I really can't imagine a good use case for it, though. But then again, until I filed this ticket, I didn't know that append-next-kill could sometimes prepend instead of append. It seems a small miracle that I've never stumbled across the prepending function by accident.
Perhaps kill-whole-line does technically kill both forwards and backwards, but to me it's always been just a welcome shortcut for the classic Emacs idiom C-a C-k C-k. And the name kill-whole-line certainly implies to me that the line is killed as a single unit, not killed in two steps in opposite directions. If the current behavior is to stay, then I think it could stand to be called out explicitly in the documentation for kill-whole-line.
Anyway! Although I'd prefer to see what I'd consider to be the more sensible behavior built into Emacs, I can achieve it on my own by just rebinding C-S-backspace to a command that moves to the beginning of the line before calling kill-whole-line.