This module provides a twig function called is_laguage_rtl which can be used by theme developer in html.twig files.
Before Drupal 8, theme developers simply would put STYLE-rtl.css files in their style directory and Drupal would handle of adding styles to page whenever site language was RTL but this feature was removed from drupal 8 above as Drupal stopped dealing with styles and scripts and started to deal with Libraries instead.