A must-use plugin is the only code that cannot be deactivated

Registering a post type in functions.php ties the content model to the theme, so changing the appearance of a site deletes its URLs.

// wp-content/mu-plugins/turkerdev-content.php
// loads first, cannot be deactivated, has no activation hook

add_action( 'init', function () {
    register_post_type( 'case_study', array( /* ... */ ) );
    register_taxonomy( 'sector', 'case_study', array( /* ... */ ) );
} );

Loading before regular plugins is the property that makes must-use right for anything the site cannot function without, including a kill switch or an environment guard. Only files at the top level of the directory load automatically, so a plugin in a subdirectory needs a one-line loader next to it — which is the detail that makes people think must-use plugins do not work. There is no update mechanism and no activation hook, so anything needing a table has to check for it on load.