Problem/Motivation
Remove deprecated code from the migration system
d6/Comment
d6/UrlAliasLanguage
d6/ViewMode
field/NodeReferemce
destination/EntityContentBase
ReviewForm.php
Remove migrate_drupal_multilingual module - this module was always intended to have a short life span. It is currently marked experimental and has been hidden since 8.9.x and there was a post update hook to uninstall it.