To access the deleted tickets on the helpdesk, navigate to the Tickets section and click on the hamburger icon and in there, Choose Trash (under the "List View Names" dropdown). You can also find the conversations from a deleted user in the Spam folder. 

You can Restore, Not spam or Delete Forever those listed tickets, under these ticket views.

Also, tickets in the trash folder for more than 30 days will be permanently removed from the portal.