The Slack integration is using Simple formatting.

This is marked by Slack themselves as deprecated (or at least most hints dropped say: Format using Block Kit).

The new way to go seems to be Block-Kit.

In OpenNMS I have seen issues with links.

For example a definition like this:

<${requisition:configlogs}|Logs>

Will just show the word Logs in Slack without a hyperlink (even if this MetaData contains an URL).
But I’ve seen also cases where it works. This is strange somehow, but I wasn’t able to figure out more. But I had contact with the Slack support and they say that we should make sure to use the Block-Kit way instead of the deprecated way.

The mentioned issue I’ve seen is of course not super important, but supporting the old deprecated way might not work for the future.

This is more or less a copy of https://github.com/prometheus/alertmanager/issues/2217 They have similar issues