Drupal distributions are specialized releases for Drupal that derive their name from Linux distributions, as they are built upon the same principles.
Top Drupal 9 Distributions
There have only been a few Drupal distributions released for Drupal 9 that are actively maintained with at least maintenance fixes only development status. This is most likely due to the fact that Drupal 8’s use of Composer to manage files and Drupal’s configuration management system have solved many of the use cases for a Drupal distribution.
Thunder is a Drupal distribution developed for professional web content publishing that includes publisher-centeric modules that have been handpicked by the Thunder Coalition. Thunder also includes custom enhancements to those modules with an environment to easily install, deploy, and add new functionality to the site.
History of Drupal Distributions
Drupal distributions are Drupal Core installations at the base level, but they are also packaged with other contributed Drupal modules, typically some custom Drupal modules to add specific functionality for the distribution, and a possibly a custom Drupal theme to bring it all together visually. There have been some amazing Drupal distributions that have come and gone over the years that have helped shape some features that have made it into Drupal Core.