See last link for the details about the following highlights:
- Lots and lots of internal changes.
- Refactored dispatch to be based on Symfony Console.
- Legacy commandfiles no longer load. See this blog post.
- Commandfiles and site alias files are not discovered in global locations by default. To use them, add a Drush config file (drush.yml) or use --include/--alias-path at the CLI. Drush config files are discovered in global locations.
- Changes since 9.0.0-beta4.