TYPO3 Core

The function returns an array of arrays with following structure:

[
    'uid' => 0,
    'title' => $this->getDefaultLanguageLabel($modSharedTSconfig),
    'ISOcode' => 'DEF',
    'flagIcon' => $this->getDefaultLanguageFlag($modSharedTSconfig),
]

The ISOcode field was used for the MultiLanguage FlexForms which support was removed with introduction of the new Backend FormEngine
The ISOcode field gets filled if static_info_tables is in use, but it isn't filled with the new introduced sys_language.language_isocode

As the field ISOcode isn't used anymore inside core, I would vote for removing this field (without deprecation) in v9LTS, because:

• Core doesn't use it anymore
• TemplaVoilà! Plus uses its own implementation
• It do not work without static_info_tables