Adding an attachment in the ticket with status as closed is possible.
Reproducing the Issue
Go to the Service work center.
Go to the Tickets view.
Search with the ticket id LMN (where LMN is the ticket with status as closed) and open it.
Go to the Attachment facet.
You can see it is allow to add a new attachment in the closed ticket LMN.
As per business process wise it's not everything in ticket is stopped.For e.g. Adding an attachment, Creating copy and Follow-up activities are still allowed.
This is an expected system behavior.
Adding Attachment , Ticket status closed , KBA , adding attachment in closed ticket , LOD-CRM-SRP , Service Request Processing , How To