Problem/Motivation
Following up from #2669964: Migrate Drupal 7 core node translations to Drupal 8, there are lots of related data to nodes that are now united in the migration to the same nodes. Say nodes 6, 7, 8, 9 and 10 all get migrated to node/6 (because 7, 8, 9, 10 are all translations of node 6), this leaves 404s at node/7, node/8, /node/9 and /node/10. Their respective path aliases are migrated to node 6 with #2850984: Fix path alias migration of translated nodes [D7] and #2827644: Fix path alias migration of translated nodes [D6].
However there is likely other related data (entity references, menu items, etc) that is not updated (yet?). This is arguably data integrity for dead links in content etc., so making this major and 'migrate critical' to at least document if not fix.
Proposed resolution
We don't have path_redirect in core, and aliases probably aren't the right fix here (to alias the node/9 path itself to node/6), but for example it would be possible to keep an nid->tnid map somewhere as part of the migration and have a listener that looks for 404s, consults the URL vs. the map and redirects if it finds a match (in an optional module).
Remaining tasks
Discuss. Implement.
User interface changes
None.
API changes
None anticipated.
Data model changes
None anticipated.