MediaWiki-extensions-Newsletter does not require the Flow extension (StructuredDiscussions), but tests are failing when Flow is not loaded:
There were 4 errors: 1) NewsletterAPIEditTest::testUpdateDescription Error: Class "MediaWiki\Extension\Notifications\Model\Event" not found 2) NewsletterAPIEditTest::testUpdateMainPage Error: Class "MediaWiki\Extension\Notifications\Model\Event" not found 3) NewsletterAPIEditTest::testAddPublisher Error: Class "MediaWiki\Extension\Notifications\Model\Event" not found 4) NewsletterAPIEditTest::testRemovePublisher Error: Class "MediaWiki\Extension\Notifications\Model\Event" not found
Those tests should use $this->markTestSkippedIfExtensionNotLoaded( 'Echo' ) or alternatively Newsletter can be marked to require Echo in extension.json.