/lib/partman/mount.d/70btrfs is in ubiquity 2.10.16.
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 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40  | #!/bin/sh
set -- $1
fs=$1
mp=$2
type=$3
options=$4
dump=$5
pass=$6
case $type in
    btrfs)
	options="${options%,subvol=*}"
	#for removing the option subvol,when thats the only option
	#eg: options=="subvol=@", no comma present
	options="${options%subvol=*}"
	mount -t btrfs ${options:+-o "$options"} $fs /target$mp || exit 1
	case $mp in
	    /)
		btrfs subvolume create /target$mp/@
		chmod 755 /target$mp/@
		umount /target$mp
		options="${options:+$options,}subvol=@"
		mount -t btrfs -o $options $fs /target$mp
		;;
	    /home)
		btrfs subvolume create /target$mp/@home
		chmod 755 /target$mp/@home
		umount /target$mp
		options="${options:+$options,}subvol=@home"
		mount -t btrfs -o $options $fs /target$mp
		;;
	esac
	echo "umount /target$mp"
	exit 0
	;;
esac
exit 1
 |