/usr/share/initramfs-tools/scripts/local-premount/btrfs is in btrfs-progs 4.7.3-1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 | #!/bin/sh
set -e
PREREQ=""
prereqs()
{
	echo "${PREREQ}"
}
case "${1}" in
	prereqs)
		prereqs
		exit 0
		;;
esac
if [ -x /bin/btrfs ]
then
	modprobe btrfs
	# If asked to be quiet, show only errors
	[ "${quiet}" = "y" ] && exec >/dev/null
	/bin/btrfs device scan
fi
 |