What is the purpose of Translation field in form display mode?
This is an extra field (Drupal term for a pseudo field not stored in the database you can move around in UI), which is added when Content Translation is enabled:  content_translation_entity_extra_field_info()

See this issue:


  content_translation_entity_extra_field_info is obsolete
  
  Problem/Motivation
  
  content_translation_entity_extra_field_info is adding the extra field
  "Translation" but it is not used by content_translation in the form
  alter hooks and it only confuses when we are on the form display
  configuring the fields and reordering it doesn't change anything on
  the edit page.
  
  Proposed resolution
  
  Remove content_translation_entity_extra_field_info
  and remove the "translation" component from all saved form display
  configs.


https://www.drupal.org/project/drupal/issues/2856551