Q:

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composer outdated "symfony/*"

Clearly, there are symfony components that are outdated. 

Is it possible to update these "outdated" components?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?

A:

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). 

