Live "What is CiviCRM?" doesn't list Backdrop, but Github does list Backdrop
Not 100% sure if this is the right place, but there seems to be an odd difference between what's live on the docs site and the GitHub repository. GitHub lists all four possible CMS choices, but the live site is missing Backdrop.
https://docs.civicrm.org/user/en/latest/introduction/what-is-civicrm/
Live:
GitHub:
https://github.com/civicrm/civicrm-user-guide/blob/master/docs/introduction/what-is-civicrm.md