As you may know, we are planning to release Drupal 10 in 2022 (as early as June), because Drupal 9's Symfony 4 and CKEditor 4 are both end of life the year after, around the end of 2023. So we plan go give enough time for people to update to Drupal 10 before Drupal 9 goes end of life. A similar situation happened with Drupal 8 to 9 driven by Symfony 3 to 4. However, moving Drupal 10 from Symfony 4 to 5 would again only give us a couple years of time to move on to Symfony 6 next, so the current plan is to move to Symfony 6 straight.