This annoying feature has appeared sometime in the past few months.
In this thread https://community.atlassian.com/t5/Jira-Software-questions/Were-there-changes-to-Jira-Cloud-app-for-Slack-for-Guest/qaq-p/1188605 there is discussion of similar things and Atlassian says "This is new functionality we've just shipped". So something has changed recently.
The issue:
When ever a message is posted to Slack, which contains a link to a Jira issue, Jira Cloud ALWAYS nags about linking the Jira account so that it could show a preview. I do not want Jira issue previews in all channels.

How do I get rid of that nagging message which fills the channels?
I still want to use the Jira-Slack integration to get specific items to a specific channel - this is working as it should.