It's do-able! The Slack UID of a user needs to be saved as a property on the Jira side so that Automation can pull it out as a smart value. This takes a little prep work!
We've provided a python script and instructions to match a Jira account and Slack account and then save the value in Jira. Following that article, you can get the properties matched and use a smart value to mention a user from Automation.
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