You can't butcher tame animals that die of other causes.
What? Cause in 40d my butchers would grab all their little parts and butcher em right up. I have not recreated it in the new version yet as I am waiting for certain fixes.
In 40d, and s far as I can tell in 31.0x, tame animals that died of any cause other than being deliberately slaughtered at a butcher's shop cannot be butchered. Wild animals can be butchered no matter how they died, but tamed animals killed by falls or any other cause won't be butchered.
Also, this:
30 z-level shaft, and at the top many female animals and one or two male animals are tied up against the walls. There is no exit. The floor is dotted with 5 floor hatches linked to a pressure plate in the center.
When a baby wanders over the pressure plate, food drops 30 z-levels and explodes on impact in a room next to my butcher shop.
won't work. The delay of a pressure plate opening a floor hatch is zero. The animal will step on the pressure plate, causing the floor hatch to instantly open before being stepped on. You need to use a retracting bridge, which has a 100 step delay between pressure plate and bridge retraction.
I have built real working autobutchers. I worked through a lot of trial and error to figure out how to get the design to actually work, a lot of the things that people think will work didn't. You need to use wild animals, because tame animals that die in falls won't be butchered. You can use bridges triggered by pressure plates for the drop, although my design simply used an auto-repeater to make the kill bridge cycle on and off constantly. I did use pressure plates to distinguish between adult and baby animals, and relied on the fact that floor hatches open instantly to make corridors that would block adult animals from leaving by having a floor hatch just following a pressure plate.