There isn't a way to restore the defect suites but deleting a Defect Suite does not delete the defects. Only the association between the Defect Suite and defects are removed and since the defects are not deleted that is why you don't see them in the recycle bin.
You can use our lookback API to find the defects that were attached to the Defect Suite here: https://ca-broadcom.wolkenservicedesk.com/external/article?articleId=212438