Cómo evitar que se publiquen posts basado en un campo personalizado

Con este snippet podemos comprobar el valor de un custom field y mostrar un error en base al mismo.

<?php
add_action( 'pre_post_update', 'intercept_publishing', 10, 2 );

function intercept_publishing( $post_ID, $data ) {
		$post = get_post( $post_ID );

	if ( ! is_admin() ) {
		return;
	}

	if ( 'post' !== get_post_type( $post_ID ) ) {
		return;
	}

	//$_REQUEST is for quick edit assuming that the post meta field was added to the quick edit view by you elsewhere
	if ( isset( $_POST['_adherence_status'] ) ) {
		$approval_status = $_POST['_approval_status'];
	} elseif ( isset( $_REQUEST['adherence_status'] ) ) {
		$approval_status = $_REQUEST['approval_status'];
	}

	if ( ! isset( $approval_status ) ) {
		return;
	}

	$post_status   = isset( $data['post_status'] ) ? $data['post_status'] : get_post_status( $post_ID );
        $edit_type     = isset( $data['post_status'] ) ? 'regular' : 'quick';
	$error_message = '';

	if ( ( $post_status === 'publish' ) && ( $approval_status !== 'approved' ) ) {
		//different messages for regular edit vs quick edit
		if ( $edit_type === 'regular' ) {
			$error_message = 'Please check the "Approval Status" field before publishing. Posts should only be published if approved by the team editor.';
		} else {
			$error_message = 'Please check the "Approval Status" and "Status" fields before clicking "Update". Posts should only be published if approved by the team editor. ';
		}

		wp_die( '<b>Publishing Error: </b> ' . $error_message, 'Adherence Publishing Error', [ 'back_link' => true ] );
	}
}