That's very interesting. I've sort of compromised on this one (for slaughter for food) by removing feed but not grass the evening before and not removing water at all. I did this because of reading a wide array of opinions on here and seeing that a more limited fast (or none at all) seems to work fine for a significant number of people. I've had no problems so far with this. In addition it makes the crop easier to locate if it has some grass in it.
As for caponing, it's not slaughter (hopefully), but a fairly minor surgical procedure. The fast is needed for the surgery, to shrink the intestines since they would be in the way otherwise.
I fasted the roosters only overnight for two reasons. First, because I couldn't be sure of the weather and didn't want to stress them unnecessarily in case I didn't end up processing them on schedule. (Which for two of them did end up happening.) Second, because I intended to kill them, THEN practice caponing on the carcasses as a way to educate myself, then after that, to process them for consumption. So, length of the procedure in this case didn't matter since they would be dead already and not suffering.
So, when I do caponize live cockerels, I will absolutely follow the 36 hr feed fast and 24 hr water fast as
@Kabootar recommends. Also thanks for the tip about a tear mending more rapidly than a clean-cut incision. I knew that fact concerning tears in childbirth, but neglected to connect it to the caponing incision. I will remember. And yes, I agree my rooster had sour crop.

Poor little guy.