functions.php

/**
 * Handles the bulk action.
 */
function my_bulk_action_handler( $redirect_to, $action, $post_ids ) {
	if ( $action !== 'draft_posts' ) {
		return $redirect_to;
	}

	foreach ( $post_ids as $post_id ) {
		wp_update_post( array(
			'ID'          => $post_id,
			'post_status' => 'draft',
		) );
	}

	$redirect_to = add_query_arg( 'bulk_draft_posts', count( $post_ids ), $redirect_to );

	return $redirect_to;
}
add_filter( 'handle_bulk_actions-edit-post', 'my_bulk_action_handler', 10, 3 );