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In Drupal 8.x hiding the subject line is a quick task.

In the admin area of your site go to structure > comment types > {comment type} > manage form display and the URL will be like https://domain.com/admin/structure/comment/manage/{comment}/form-display.  I have put the type in braces as you might have several comment types.  If you only have the default Drupal setting, then use default comments and the path will be https://domain.com/admin/structure/comment/manage/comment/form-display

Action

Drag the subject to the disabled area and save.