Problem/Motivation
Dependencies of Drupal such as Symfony, Twig, Guzzle, etc. may become unsupported in Drupal 8's lifetime or Drupal 9's expected lifetime.
Proposed resolution
Update dependencies them to their latest (or other appropriate) major versions as applicable. Open child issues for each update.
Remaining tasks
See above.
User interface changes
Likely none, depends on updates with UIs.
API changes
Depend on updated dependencies that Drupal is exposing as-is.
Data model changes
Depend on updates.