WordPress 4.4 loads a few new files in
wp-settings.php relating to oEmbed and the REST API. Because WP-CLI has a custom
wp-settings-cli.php (background), WP-CLI v0.20.3 is a compatibility release to load these new files.
Importantly, due to the nature of these changes, WP-CLI versions prior to 0.20.3 will be incompatible with WordPress 4.4.
Stay tuned next week for WP-CLI v0.21.0 (which is also compatible with WordPress 4.4), the results of the user survey, and a special announcement.
You can browse the full list of resolved issues on Github.