b2evolution

 b2evolution  is an open source blogging/CMS platform, one of the original surviving forks of the B2 blog engine, and added Webmention support 2019-03-22. 

IndieWeb Support

 * Webmention: Discussion on Ramblinggit.com (in comments) where B2evolution maintainer noted that they "have added Webmentions support to the todo list".
 * Update: 2019-03-22 B2evolution adds Webmentions support to version 6.11.0-beta
 * Update 2019-12-15 Brad Enslen confirms that it sends/receives Webmention.