Currently MediaWiki contains deprecated language codes:
- LanguageCode::DEPRECATED_LANGUAGE_CODE_MAPPING (https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/core/+/master/includes/language/LanguageCode.php#39) contains a mapping from deprecated language codes to the actual used language codes.
- MediaWiki\Languages\Data\Names::$names (https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/core/+/master/languages/data/Names.php#48) contains duplicate entries for deprecated language codes with the same value as the actual used language codes.
The deprecated language codes are only used in WMF projects. These language codes are not needed in MediaWiki itself. Some language codes even block a standardized use of the language codes (als is the language code for Tosk Albanian and not for Alemannic) in MediaWiki. Therefor the mapping LanguageCode::DEPRECATED_LANGUAGE_CODE_MAPPING and the deprecated language codes in Names.php should moved from MediaWiki to a WMF specific configuration.