In Drupal 7, modules implementing entity types had to call field_attach_preprocess() in their hook_ENTITY_TYPE_preprocess() implemantations.
This made the raw field values in the displayed language, which were otherwise not easily accessible due to the complex multilingual fallback logic, available in the corresponding templates (node.tpl.php, taxonomy_term.tpl.php...) as direct variables, named after the the field name.
In Drupal 8, the new Entity Translation API greatly simplified the logic to access field values.
The raw field values in the correct language are available directly through the template variable representing the entity : node.field_foo
, term.field_bar
...
The field_attach_preprocess() preprocess function has thus been removed. This also includes the removal of the hook hook_field_attach_preprocess_alter()
.