To be able to archive or delete a subject, you must be an officer of the subject or a workspace admin.
Once you have the rights, click on the three dots on the right of the subject's name, and choose “Archive this subject” :
A confirmation window will appear. You can optionally check the box to move all remaining tasks in the subject to the Canceled state.
An archived subject can be easily identified with its "Archived" tag next to its name.
Archived subjects will be removed from the side menu and will not appear in the search results. You can find all archived subjects in the subject index:
You can reopen an archived subject by clicking on the three dots and selecting "Unarchive this subject":