The Jira Service Desk app has been retired and incorporated into the standalone Jira Service Desk application. If you're running Jira Server 6.x or earlier, you can download a past version of the app from the list below and manually install it on your server.
Version 2.5.9 • Released 2015-10-13 • Supported By Atlassian • Paid via Atlassian • Atlassian Closed-Source
https://confluence.atlassian.com/display/SERVICEDESK/JIRA+Service+Desk+2.5.9+Release+Notes