/lib/partman/init.d/50biosgrub 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  | #! /bin/sh
# Set method "biosgrub" for all partitions that have the bios_grub flag set.
. /lib/partman/lib/base.sh
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
		if [ "$fs" != free ]; then
			partitions="$partitions $id"
		fi
	done
	close_dialog
	for id in $partitions; do
		biosgrub=no
		open_dialog GET_FLAGS $id
		while { read_line flag; [ "$flag" ]; }; do
			if [ "$flag" = bios_grub ]; then
				biosgrub=yes
				# cannot break here
			fi
		done
		close_dialog
		if [ "$biosgrub" = yes ]; then
			mkdir -p $id
			echo biosgrub >$id/method
		fi
	done
done
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