Backward Compatibility allows you to install Drupal 8 modules in Drupal 9.

Once you install Backward Compatibility, Drupal 9 will no longer prevent you from installing Drupal 8 modules. The module you are trying to use may still not work in Drupal 9, but Drupal will no longer stop you from doing what you tell it to do.

This module works around Drupal 8 modules that are missing this line in their info.yml files: