Nothing happens to the defect when the Assigned To user is deleted from ALM.
When a UserName field is filled in, that data is explicitly written into the corresponding field in the entity in the database. If the user is deleted from the project or the site, the text of the username remains in that field.
When a UserName field is filled in, that data is explicitly written into the corresponding field in the entity in the database. If the user is deleted from the project or the site, the text of the username remains in that field.