There is a seemingly endless stream of open source and proprietary Content Management System (CMS) to use in building a website. For a number of years I have been a more or less loyal user of the Drupal CMS. In terms of popularity of open source CMS, the common ranking is Wordpress, Joomla and Drupal. That popularity order is the reverse of the sophistication level of the three CMSs. Drupal has generally been recognized as the most sophisticated of the (popular) open source CMSs.
I was comfortable with Drupal V6 and Drupal V7, and then came Drupal V8. The V8 version introduced several new technologies. For a large, sophisticated multi-site web presence V8 promised to take web development to the next level. For a while I went along with this more sophisticated approach, but eventually I hit a brick wall. Drupal V9 didn't do what I wanted out of the box. To effect my desired changes I would be forced to work the next level down, and therein lay a major problem for me. The necessary learning curve would be steep (for me) if I stayed with Drupal and moved to V9.
But there is an alternative. A number of Drupal V7 developers recognized the need for incremental improvement, but didn't see the need to add a basket of sophisticated new technologies. The Backdrop CMS was born as a fork of Drupal V7. Backdrop is very unlikely to ever be as popular as Drupal. Developers come from the ranks of web builders who are pushing the technology. But Backdrop has more modest goals. It wants to be the practical alternative for smaller, less ambitious sites. It's now been available for some five years. It continues to improve, providing a web development platform that fits the need to the rest of us.
I would not argue that Backdrop CMS is the web development platform that (almost) everyone should use. It does make sense for those of us concerned about a high quality web presence that needs only a modest investment in the under-girding technologies. I recommend it be given serious consideration by all those interested in a high quality web development platform built from relatively simple parts.