Script, to be able to be run by Monit, needs to return exit code instead of sending an email. Next difference is that in bellows script lines 126 and 127 are uncommented (and lines 130 and 131 commented). This is to support Ubuntu date format. If you want to use this script on FreeBSD then you want these lines to look like in the original script. Next thing is that, in the beginning of the script, I added support for input parameters for max capacity and scrub expiration of your zpools. This is in order to keep configuration in one place (i.e. Monit's configuration file). Last but not least is the "Output for monit user interface" section. As its title says it outputs zpool status to the console so it could be recorded by Monit and displayed in its user interface. 

#! /bin/bash
#
## ZFS health check script for monit.
## v1.0.2
#
## Should be compatible with FreeBSD and Linux. Tested on Ubuntu.
## If you want to use it on FreeBSD then go to Scrub Expired section and Trim Expired section
## and comment two Ubuntu date lines and uncomment two FreeBSD lines in Scrub Expired section.
## In Trim Expired section adjust the date format directly in the for loop's awk parameter.
#
## Assumed usage in monitrc (where 80 is max capacity in percentages
## and 691200 is scrub and trim expiration in seconds):
## check program zfs_health with path "/path/to/this/script 80 691200"
##     if status != 0 then alert
#
## Scrub and Trim share the same expiration threshold for the backward compatibility reasons.
#
## Original script from:
## Calomel.org
##     https://calomel.org/zfs_health_check_script.html
##     FreeBSD ZFS Health Check script
##     zfs_health.sh @ Version 0.17
#
## Main difference from the original script is that this one exits
## with a return code instead of sending an e-mail

# Parameters

maxCapacity=$1 # in percentages
scrubExpire=$2 # in seconds (691200 = 8 days)
trimExpire=$2 # in seconds (691200 = 8 days)

usage="Usage: $0 maxCapacityInPercentages scrubExpireInSeconds\n"

if [ ! "${maxCapacity}" ]; then
  printf "Missing arguments\n"
  printf "${usage}"
  exit 1
fi

if [ ! "${scrubExpire}" ]; then
  printf "Missing second argument\n"
  printf "${usage}"
  exit 1
fi


# Output for monit user interface

printf "==== ZPOOL STATUS ====\n"
printf "$(/sbin/zpool status)"
printf "\n\n==== ZPOOL LIST ====\n"
printf "%s\n" "$(/sbin/zpool list)"


# Health - Check if all zfs volumes are in good condition. We are looking for
# any keyword signifying a degraded or broken array.

condition=$(/sbin/zpool status | grep -E 'DEGRADED|FAULTED|OFFLINE|UNAVAIL|REMOVED|FAIL|DESTROYED|corrupt|cannot|unrecover')

if [ "${condition}" ]; then
  printf "\n==== ERROR ====\n"
  printf "One of the pools is in one of these statuses: DEGRADED|FAULTED|OFFLINE|UNAVAIL|REMOVED|FAIL|DESTROYED|corrupt|cannot|unrecover!\n"
  printf "$condition"
  exit 1
fi


# Capacity - Make sure the pool capacity is below 80% for best performance. The
# percentage really depends on how large your volume is. If you have a 128GB
# SSD then 80% is reasonable. If you have a 60TB raid-z2 array then you can
# probably set the warning closer to 95%.
#
# ZFS uses a copy-on-write scheme. The file system writes new data to
# sequential free blocks first and when the uberblock has been updated the new
# inode pointers become valid. This method is true only when the pool has
# enough free sequential blocks. If the pool is at capacity and space limited,
# ZFS will be have to randomly write blocks. This means ZFS can not create an
# optimal set of sequential writes and write performance is severely impacted.

capacity=$(/sbin/zpool list -H -o capacity | cut -d'%' -f1)

for line in ${capacity}
  do
    if [ $line -ge $maxCapacity ]; then
      printf "\n==== ERROR ====\n"
      printf "One of the pools has reached it's max capacity!"
      exit 1
    fi
  done


# Errors - Check the columns for READ, WRITE and CKSUM (checksum) drive errors
# on all volumes and all drives using "zpool status". If any non-zero errors
# are reported an email will be sent out. You should then look to replace the
# faulty drive and run "zpool scrub" on the affected volume after resilvering.

errors=$(/sbin/zpool status | grep ONLINE | grep -v state | awk '{print $3 $4 $5}' | grep -v 000)

if [ "${errors}" ]; then
  printf "\n==== ERROR ====\n"
  printf "One of the pools contains errors!"
  printf "$errors"
  exit 1
fi


# Scrub Expired - Check if all volumes have been scrubbed in at least the last
# 8 days. The general guide is to scrub volumes on desktop quality drives once
# a week and volumes on enterprise class drives once a month. You can always
# use cron to schedule "zpool scrub" in off hours. We scrub our volumes every
# Sunday morning for example.
#
# Check your /etc/cron.d/zfsutils_linux for any already scheduled jobs
#
# Scrubbing traverses all the data in the pool once and verifies all blocks can
# be read. Scrubbing proceeds as fast as the devices allows, though the
# priority of any I/O remains below that of normal calls. This operation might
# negatively impact performance, but the file system will remain usable and
# responsive while scrubbing occurs. To initiate an explicit scrub, use the
# "zpool scrub" command.
#
# The scrubExpire variable is in seconds.

currentDate=$(date +%s)
zfsVolumes=$(/sbin/zpool list -H -o name)

for volume in ${zfsVolumes}
  do
    if [ $(/sbin/zpool status $volume | grep -E -c "none requested") -ge 1 ]; then
      printf "\n==== ERROR ====\n"
      printf "ERROR: You need to run \"zpool scrub $volume\" before this script can monitor the scrub expiration time."
      break
    fi

    if [ $(/sbin/zpool status $volume | grep -E -c "scrub in progress|resilver") -ge 1 ]; then
      break
    fi

    ### Ubuntu with GNU supported date format - compatible with ZFS v2.0.3 output
    scrubRawDate=$(/sbin/zpool status $volume | grep scrub | awk '{print $11" "$12" " $13" " $14" "$15}')
    scrubDate=$(date -d "$scrubRawDate" +%s)

    ### FreeBSD with *nix supported date format
    #scrubRawDate=$(/sbin/zpool status $volume | grep scrub | awk '{print $15 $12 $13}')
    #scrubDate=$(date -j -f '%Y%b%e-%H%M%S' $scrubRawDate'-000000' +%s)

    if [ $(($currentDate - $scrubDate)) -ge $scrubExpire ]; then
      printf "\n==== ERROR ====\n"
      printf "${volume}'s scrub date is too far in the past!"
      exit 1
    fi
  done

# TRIM Expired - Check if all volumes have been trimmed in at least the last
# 8 days. The general guide is to manually trim volumes on desktop quality drives once
# a week and volumes on enterprise class drives once a month. You can always
# use cron to schedule "zpool trim" in off hours. We trim our volumes every
# Sunday morning for example.
#
# Check your /etc/cron.d/zfsutils_linux for any already scheduled jobs
#
# Manual trimming is recommended even though autotrim feature is turned on for your pool.
# From ZFS documentation:
# > Since the automatic TRIM will skip ranges it considers too small there is value in occasionally
# > running a full zpool trim. This may occur when the freed blocks are small and not enough time
# > was allowed to aggregate them. An automatic TRIM and a manual zpool trim may be run concurrently,
# > in which case the automatic TRIM will yield to the manual TRIM.

for volume in ${zfsVolumes}
  do
    if [ $(/sbin/zpool status -t $volume | grep -E -c "trim unsupported") -ge 1 ]; then
      break
    fi

    ### Ubuntu with GNU supported date format - compatible with ZFS v2.0.3 output - For other systems and version adjust awk parameter below
    trimRawDates=$(/sbin/zpool status -t $volume | grep trim | awk '{print $10" "$11" " $12" " $13" "$14}')

    while IFS= read -r trimRawDate
      do
        trimDate=$(date -d "$trimRawDate" +%s)

        if [ $(($currentDate - $trimDate)) -ge $trimExpire ]; then
          printf "\n==== ERROR ====\n"
          printf "${volume}'s trim date is too far in the past!"
          exit 1
        fi
      done <<< "$trimRawDates"
  done

# Finish - If we made it here then everything is fine
exit 0
Direct link to the script file

To use the script go to your monitrc file and add following lines: 

check program zfs_health with path "/path/to/this/script 80 691200"
  if status != 0 then alert
Where 80 is the max capacity in percentages and 691200 is the scrub expiration in seconds. This will make Monit notify you everytime something is wrong with your zpool.

As a bonus you will get nice status with last script run output on the web user interface:

ZFS health check status on the web user interface
Figure 1. ZFS health check status on the web user interface

If you don't see the whole output for your zpools then you probably need to set higher PROGRAMOUTPUT limit in monitrc file. The default one is 512 bytes.

The script is also available as Github Gist.