you can delete/update already posted alarms in slack
you can join alarms (problem, acknowledgements, resolve) together using Slack threads/
you can post raw JSON in Zabbix to Slack API to fine tune formatting in Slack...
this script forks by defatul to avoid overwhelming synchronous zabbix alerter process.(especially actual for Zabbix ver < 3.4)....
See README for all features.
This other solution seems so much simpler: https://aperogeek.fr/zabbix-send-alerts-to-slack/
What are the benefits of zabbix-notify?