Triggered by https://opennms.atlassian.net/browse/NMS-15652
Slack deprecated the old way to use the (old) incoming web hook feature that allowed defining different channels.

This issue here is about finding the best solution to send messages into Slack, and especially how to handle different Slack chat channel in OpenNMS.

The new way now is, to use a so called Slack app that provides again a web hook feature. The big difference now is, that this slack app - web hook is set to one specific Slack channel. For me it looks like it is not possible to overwrite the channel. So we need one Slack app with a unique web hook per Slack channel that ONMS wants to feed. This means for OpenNMS, that we have to create one destination path using one specific notification command with the corresponding web hook for -one- channel. There is another issue about making meta-data in destination paths, that could make the current situation a bit easy to handle.
This is not quite handy if you want to send notifications into multiple channels.

Another way seems to be sending messages to Slack is using the chat API postMessage.
Regarding this documentation the channel can be dynamically set per call.
Maybe this way is the better one for OpenNMS?