Problem/Motivation
PHPUnit 10 was originally planned for February 2021, but has been delayed. We are hoping to ship Drupal 10 with support for PHPUnit 10 and so we can start preparing for this now, even though it is not fully released yet, thanks to Symfony's PHPUnit bridge and #3174200: Use PHPUnit-bridge polyfills for forward compatibility layer which adds forward compatibility for PHPUnit 10 to existing tests.
Proposed resolution
Start converting the deprecated methods as possible- Obviously, wait for PHPUnit 10 to be a thing
- The biggest effort will then come, i.e. converting the deprecated listeners (that will be removed in PHPUnit 10 AFAWCS) to the new event system included in PHPUnit 10. This will have upstream impacts on Symfony's PHPUnit-bridge, too, because the deprecation reporting process that it (and from there, Drupal) implements is based on listeners. So we likely will have to see what Symfony startegy will be before outlining ours.
Remaining tasks
#3125809: Remove usages of \PHPUnit\Framework\TestListener that is deprecated
+ changing to the world order of the event system.