Matt Mullenweg's "A Plan for 5.0" is actually a list about "several members of the WordPress community," primarily his Automattic employees, assigned to the current team. Where's the plan? For that look here:
https://make.wordpress.org/core/5-0/

This formal Release Schedule linked from the WordPress.org Roadmap is a table of steps including four Betas without any dates, just "TBD."

With such a major shift as WordPress 5 will bring, it'd be useful to have full details on WordPress.org. Here's one example of the various roadmaps for Drupal: