/lib/partman/veto_filesystems/crypto is in ubiquity 2.18.7.
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 | #!/bin/sh
# Veto filesystems unsuitable for certain crypto setups
dev=$1
id=$2
filesystems="$3"
veto_filesystems ()
{
	[ -f $dev/crypt_realdev ] || return 1
	# Get to the underlying crypto device directory
	r=$(cat $dev/crypt_realdev)
	cryptodev=${r##*:}
	[ -f $cryptodev/method ] || return 1
	method=$(cat $cryptodev/method)
	if [ $method = crypto ]; then
		[ -f $cryptodev/keytype ] || return 1
		keytype=$(cat $cryptodev/keytype)
		if [ $keytype = random ]; then
			# Veto anything but ext2
			for fs in $filesystems; do
				case $fs in
				    ext2) echo $fs ;;
				esac
			done
			return 0
		fi
	fi
	return 1
}
veto_filesystems || echo $filesystems
exit 0
 |