When the Date component was added to core in #1802278: Add a Date component to core, it included support for the IntlDateFormatter feature of PHP 5.3+. This class provides locale-aware date formatting for PHP dates. When a Drupal site is running PHP is compiled with INTL support (http://ca2.php.net/manual/en/intl.installation.php), DrupalDateTime::format() would attempt to automatically switch to using IntlDateFormatter instead of the date_format() function used in Drupal 7.
However, the feature resulted in numerous critical bugs and regressions related to dates, and the automatic switching was not well supported or tested. Therefore, support is being removed.
Date formats can still be localized and translated as they were in Drupal 7 and earlier following the change. Support for INTL dates may be added back in a future release if the related issues are resolved.
Comments
https://www.drupal.org
https://www.drupal.org/project/intl_date - if you need the functionality of Intl extension for any reason, it is available as a contrib module what tries to be a drop-in replacement of the core date formatting.