/lib/partman/finish.d/70aptinstall_btrfs 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  | #!/bin/sh
. /lib/partman/lib/base.sh
btrfs=no
for dev in $DEVICES/*; do
	[ -d "$dev" ] || continue
	cd $dev
	partitions=
	open_dialog PARTITIONS
	while { read_line num id size type fs path name; [ "$id" ]; }; do
		[ "$fs" != free ] || continue
		[ -f $id/method -a -f $id/acting_filesystem ] || continue
		filesystem=$(cat $id/acting_filesystem)
		case $filesystem in
		    btrfs)
			btrfs=yes
			;;
		esac
	done
	close_dialog
done
if [ "$btrfs" = yes ]; then
	apt-install btrfs-tools || true
fi
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