The Drupal 7 countries module → has no Drupal 8+ version. Still, this module and https://www.drupal.org/project/countries_field → seem to replicate the functionality for Drupal 8+.
So for projects upgrading from Drupal 7 it would be super helpful to provide a migration path from Drupal 7.
Implement a Drupal 7 field migration mapping from countries field to country field.
Closed: duplicate
1.0
Code
Sorry, I saw much too late that the widely used
Address module →
provides address_country field. Of course, that's the better project to join forces on! :)