There is one constant used in magento/importexport/model/import.php file.
i.e. DEFAULT_GLOBAL_MULTI_VALUE_SEPARATOR having value comma as default but to update this I override file with preference but even after that the import form not showing | as default value instead of , (comma)
The form is created from
magento/importexport/block/adminhtml/import/edit/Form.php
Where a text field
'name' => Import::FIELD_FIELD_MULTIPLE_VALUE_SEPARATOR
use
'value'=> Import::DEFAULT_GLOBAL_MULTI_VALUE_SEPARATOR
Do we need to override this file as well using preference as this is a protected method _prepareForm