WordPress 4.4 loads a few new files even more files in wp-settings.php. Because WP-CLI has a custom wp-settings-cli.php (background), WP-CLI v0.21.1 and v0.20.4 are compatibility releases to load these new files.
Importantly, due to the nature of these changes, WP-CLI versions prior to 0.20.4 will be incompatible with WordPress 4.4.
Inspect the nature of the change in this pull request.
Contributors to this release: danielbachhuber