Yaourt on archlinux changed AUR behavior.

yaourt -Syu --aur used to show aur packages, recently it stopped.

The fix is to replace the line

DETAILUPGRADE=0 classify_pkg $(pacman -Qqm | wc -l)< <(eval $cmd)

to

classify_pkg < <(eval $cmd)

in /usr/lib/yaourt/abs.sh

EDIT:

This is now accomplished with

DETAILUPGRADE=2 in /etc/yaourtrc