Hi,
right now the menu_links migration does not handle links to nodes which are redirected because links to translations of other nodes.
I get one failure when migrating the menu links from D6, the link is dropped in the destination menu with the migration giving this message:
The path "internal:/node/2" failed validation.
I think the cause of this failure is that in the D6 site node/2 is a translation of node/1, so there is no entity with nid = 2 in the migrated D8 nodes, and thus the check in core/modules/menu_link_content/src/Plugin/migrate/process/LinkUri.php fails.
As per #2850085: Redirects for translation set migration path in Drupal 6 and 7 a redirection has been created:
SELECT * FROM `key_value` WHERE `collection` = 'node_translation_redirect' AND `name` = 2
node_translation_redirect 2 a:2:{i:0;s:1:"1";i:1;s:2:"it";}
so the route to "node/2" could resolved eventually with a local redirect, but the link_uri plugin used in the menu_links migration is checking just the Uri not the route and fails to recognize that node/2 is still usable.
Thanks,
Antonio