The older JIRA app (included in Deskpro build #383 or older) only allowed agents to export ticket information to a new JIRA issue.
It looked like this:
If you are using this older JIRA app with Deskpro On-Premise, it will be disabled when you update your helpdesk to a newer build. (Cloud users have already been updated).
There is no way to run the new app on an older build of Deskpro, or to continue using the old app on a newer build.
Installing the new app using the instructions in the JIRA app section of the admin manual will restore all your JIRA links.