After using grub-customizer, how to recover the ZFS Grub2 menu options on Ubuntu 20.04

Try opening a terminal and running

sudo chmod +x /etc/grub.d/20_memtest86+
sudo update-grub

Check to see if it's there

grep memtest /boot/grub/grub.cfg

Just done the same as I had it disabled

hob@hob-HP-dx5150-MT:~$ cat /boot/grub/grub.cfg |grep memtest
hob@hob-HP-dx5150-MT:~$ sudo chmod +x /etc/grub.d/20_memtest86+
[sudo] password for hob: 
hob@hob-HP-dx5150-MT:~$ sudo update-grub
Generating grub.cfg ...
Found linux image: /boot/vmlinuz-3.2.0-24-generic
Found initrd image: /boot/initrd.img-3.2.0-24-generic
Found linux image: /boot/vmlinuz-3.2.0-23-generic
Found initrd image: /boot/initrd.img-3.2.0-23-generic
Found memtest86+ image: /boot/memtest86+.bin
Found Ubuntu 11.10 (11.10) on /dev/sda6
Found Trisquel 5.5 (5.5) on /dev/sda8
Found Ubuntu 12.04 LTS (12.04) on /dev/sda9
done
hob@hob-HP-dx5150-MT:~$ cat /boot/grub/grub.cfg |grep memtest
### BEGIN /etc/grub.d/20_memtest86+ ###
menuentry "Memory test (memtest86+)" {
    linux16 /boot/memtest86+.bin
menuentry "Memory test (memtest86+, serial console 115200)" {
    linux16 /boot/memtest86+.bin console=ttyS0,115200n8
### END /etc/grub.d/20_memtest86+ ###

You could try creating the file if it's not there, using nano in a terminal

sudo nano /etc/grub.d/20_memtest86+

or with a gui editor

gksudo gedit /etc/grub.d/20_memtest86+

Fill it with

#!/bin/sh
set -e

if [ -f /usr/lib/grub/grub-mkconfig_lib ]; then
  . /usr/lib/grub/grub-mkconfig_lib
  LX=linux16
elif [ -f /usr/lib/grub/update-grub_lib ]; then
  . /usr/lib/grub/update-grub_lib
  LX=linux
else
  # no grub file, so we notify and exit gracefully
  echo "Cannot find grub config file, exiting." >&2
  exit 0
fi

# We can't cope with loop-mounted devices here.
case ${GRUB_DEVICE_BOOT} in
  /dev/loop/*|/dev/loop[0-9]) exit 0 ;;
esac

prepare_boot_cache="$(prepare_grub_to_access_device ${GRUB_DEVICE_BOOT} | sed -e "s/^/\t/")"

if test -e /boot/memtest86+.bin ; then
  MEMTESTPATH=$( make_system_path_relative_to_its_root "/boot/memtest86+.bin" )
  echo "Found memtest86+ image: $MEMTESTPATH" >&2
  cat << EOF
menuentry "Memory test (memtest86+)" {
EOF
  printf '%s\n' "${prepare_boot_cache}"
  cat << EOF
    $LX $MEMTESTPATH
}
menuentry "Memory test (memtest86+, serial console 115200)" {
EOF
  printf '%s\n' "${prepare_boot_cache}"
  cat << EOF
    $LX $MEMTESTPATH console=ttyS0,115200n8
}
EOF
fi

#if test -e /boot/memtest86+_multiboot.bin ; then
#  MEMTESTPATH=$( make_system_path_relative_to_its_root "/boot/memtest86+_multiboot.bin" )
#  echo "Found memtest86+ multiboot image: $MEMTESTPATH" >&2
#  cat << EOF
#menuentry "Memory test (memtest86+, experimental multiboot)" {
#EOF
#  printf '%s\n' "${prepare_boot_cache}"
#  cat << EOF
#   multiboot   $MEMTESTPATH
#}
#menuentry "Memory test (memtest86+, serial console 115200, experimental multiboot)" {
#EOF
#  printf '%s\n' "${prepare_boot_cache}"
#  cat << EOF
#   multiboot   $MEMTESTPATH console=ttyS0,115200n8
#}
#EOF
#fi

Make sure to run the chmod +x command on it afterwards.

I'd be a bit concerned that it's missing though - do you have everything else in grub.d ?

ls /etc/grub.d/


00_header        10_linux.bak       20_memtest86+  41_custom
05_debian_theme  10_linux.dpkg-old  30_os-prober   README
10_linux         20_linux_xen       40_custom

You are using lvm or btrfs for your / partition.

This is made by design, because grub can't write to either of those. This may lead to unablity to enter grub after a boot failure.

If you are sure that's not a problem for you, add GRUB_RECORDFAIL_TIMEOUT=0 to /etc/default/grub and run

sudo update-grub

Otherwise consider making a separate /boot partition formatted to ext2.