Tickets autoclose is a newly added feature to the Helpdesk plugin.
Settings
To set autoclose as Redmine administrator, go to Helpdesk plugin General settings (or find Helpdesk in the Plugins list, and select Configure). To activate the feature, set autoclose deadline.
After that, you will be able to set status change for autoclosed tickets:
Autoclose template
If you would like the client to be notified via autoclose message, then please go to Project - Settings - Helpdesk template - Autoclose template section.
The same setting could be applied also globally (for all projects) in - Administration - Helpdesk - Helpdesk template.
Note: Please keep in mind that the projects settings override the global settings.