For DrupalCon North America 2021, you can search the core issue queue with both the "NorthAmerica2021" and "Drupal 10" tags or CKEditor 5 issues in the contributed project.
After DrupalCon, you can search the core issue queue with just the "Drupal 10" tag or the general issue queue with the "Drupal 10 compatibility" tag to find Drupal 10 readiness issues.