Boldly resolving this. PHPUnit has been updated everywhere. Most places already work with the new version, including MW core and gated extensions. Many other non-gated extensions have also been updated; I should have sent patches for all extensions returned by codesearch. Yet, some of those extensions have broken CI, and this seems to have been the case for months. Patches for those extensions have not been merged, but are there in review. That's pretty much all we could do, hence closing this task.
If the update has caused breakage in an extension with otherwise healthy CI, please file a separate task and I'll look into it.