You can hand-roll this with a Slack event listener + the Jira REST API, but keeping it idempotent (no dupes on retries) and handling edits/attachments is the hard part. Disclosure: I work on Threadmaker. That two-way comment sync is exactly what we do: replies in the mapped Slack thread become Jira comments and Jira comments post back, with attachments and @mentions translated both ways. Glad to point you at it if you'd rather not maintain a script.
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