flush-rewrite-rules-on-all-sites

/*
 * This is old and doesn't always work correctly, see the following post for a better way:
 *
 * https://iandunn.name/flushing-rewrite-rules-on-all-sites-in-a-multisite-network/
 */

function flush_rewrite_rules_everywhere() {
	global $wp_rewrite;
	$sites = wp_get_sites( array( 'limit' => false ) );

	foreach ( $sites as $site_id ) {
		switch_to_blog( $site_id );
		$wp_rewrite->init();

		flush_rewrite_rules();

		restore_current_blog();
	}
	$wp_rewrite->init();
}