Problem/Motivation
The Symfony 3.4 LTS will be EOL in 2021: http://symfony.com/doc/current/contributing/community/releases.html#sche...
This means we need to be ready to move to Symfony 4 before then.
Proposed resolution
We should at least do the following:
1. Identify all the deprecated Symfony code that Drupal 8 relies on and update for it, opening issues for anything not straightforward #2959269: [meta] Core should not trigger deprecated code except in tests and during updates
2. Have an issue with a patch that updates to Symfony 4 so we can see if it passes or not. This is ongoing at #2976394: Investigate problems with Symfony4 now
3. Potentially, relax our composer.json requirements to allow Symfony 4 to be installed, once the above two are done, if it's feasible to do so.
Remaining tasks
We need to resolve https://github.com/symfony/symfony/issues/25786 upstream.