Hi @Ramesh Tathe Thank you for posting your question. Sorry for the late reply.
is 0 threat retention days means retain forever ?
Yes, ,0 =forever. I believe it is in the audit setup/ configuration. Some aspect of threat data retention is controlled by how you configure your storage account(s) where the corresponding audit events flow.
If you are you refereeing to threat detections (alerts)? Those are stored in Azure Security Center.
Hope that helps
Regards,
Oury