Symfony 4 does not contain hard breaking changes compared to Symfony 3 - at least for the components that Drupal core uses and in the way that we use them. Therefore, once the issues that do affect us are fixed (mainly the removal of Symfony's classloader), it should be possible to allow sites to run Symfony 4.
We'll still want to package core with Symfony 3, Symfony 4 would only be available installing with composer.
Postponed: needs info
11.0 🔥
base system
It is used to alert the release manager core committer(s) that an issue significantly affects the overall technical debt or release timeline of Drupal, and their signoff is needed. See the governance policy draft for more information.
It is used to alert the framework manager core committer(s) that an issue significantly impacts (or has the potential to impact) multiple subsystems or represents a significant change or addition in architecture or public APIs, and their signoff is needed (see the governance policy draft for more information). If an issue significantly impacts only one subsystem, use Needs subsystem maintainer review instead, and make sure the issue component is set to the correct subsystem.
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Now that we've done 🌱 [policy, no patch] Adopt a regular two year release cadence for major releases Fixed this is no longer necessary I think. Moving to 'needs more info' in case there's a use-case I've missed. I guess potentially it would still be possible to update the 10.x branch to newer Symfony versions using this, but any sites wanting to do that would be better off updating to 11.x anyway.