In b90593333c3b0c4221619e2c00edc514af9a22eb the class \MediaWiki\Message\Message was moved from includes/Message/Message.php to includes/language/Message/Message.php as part of T364652.
The new location does not allow the class to autoload via PSR-4 (T353458: Make core classes follow PSR-4)

The class needs to be re-namespaced (T409441: Remove hierarchy, or re-namespace various classes) to \MediaWiki\Language\Message\Message, but that break phan with a class-alias as special handling for Message class is no longer recognized by phan-taint-check.

• Verify the new namespace is the correct namespace
• Change phan-taint-check
• Release phan-taint-check
• Release mediawiki-phan-config
• Update mediawiki-phan-config everywhere
  
  => Re-namespace the class via T411216: Change namespace from \MediaWiki\Message\Message to \MediaWiki\Language\Message\Message