We have a custom php script that checks the activity table in the nextcloud mysql database and fires webhook notifications to mattermost and slack.
The nice feature is that you can map folder hierarchies to slack/mattermost channels so only relevant people get notified about changes to their shared documents. The messages contain internal links so clicking them in messaging app automatically opens the file in nextcloud.
I would like this app to be fully maintained from nextcloud UI by becoming a flow compliant custom app. Happy to share the php source code with interested parties.