Our theme and plugins still use phpunit 6, but that version is not supported anymore since over a year. The documentation for 6 has even been removed from phpunit's website.

 https://github.com/greenpeace/planet4-master-theme/blob/5da9b14c608cb0063175326708db78238fa40bf7/composer.json#L41

https://phpunit.de/supported-versions.html

Tasks

• Investigate if needed to update
  If needed:
• Update phpunit to latest version
• Make sure unit tests on CI keep running as expected
• Make sure local dev environment is not affected by these change