Update Symfony components There are many posts and articles that make vague references to upgrading Symfony 3 to Symfony 4 in Drupal 8. But nothing specific. 

For example this one by Ryan Weaver "Symfony 4 and Drupal"

I've installed a fresh copy of Drupal 8.8.1 and updated with 

 composer update drupal/core --with-dependencies


Then, I ran 

composer outdated "symfony/*"


Clearly, there are symfony components that are outdated. 



Is it possible to update these "outdated" components? Or, are we stuck till Drupal 9?
As Ryan Weaver mentions,


  The current version of Drupal - 8.4 - requires Symfony 3.2.8 or
  higher, but less than 4.0. In practice, this probably means that your
  Drupal 8.4 project is using the latest Symfony 3.2.* version. But, you
  could technically upgrade your Symfony dependencies to version 3.4.
  
  Source: Symfony 4 and Drupal


Thus, you can, but only for minor versions. 



To quote Dries Buytaert,


  Our biggest dependency in Drupal 8 is Symfony 3, and according to
  Symfony's roadmap, Symfony 3 has an end-of-life date in November 2021.
  This means that after November 2021, security bugs in Symfony 3 will
  not get fixed. To keep your Drupal sites secure, Drupal must adopt
  Symfony 4 or Symfony 5 before Symfony 3 goes end-of-life. A major
  Symfony upgrade will require us to release Drupal 9 (we don't want to
  fork Symfony 3 and have to backport Symfony 4 or Symfony 5 bug fixes).
  This means we have to end-of-life Drupal 8 no later than November
  2021.
  
  Source: Drupal 7, 8 and 9


He says here that Drupal 8 will reach end of life of Nov 2021 because that's Symfonys 3's end of life. Thus, from this, one can conclude that you can't just upgrade D8 to Symfony 4 (another major Symfony version).