flush_network_admin_bar.php

<?php
/*
 * Plugin name: Test Rewrite Plugin
 
 * Source: https://jeremyfelt.com/2015/07/17/flushing-rewrite-rules-in-wordpress-multisite-for-fun-and-profit/
 * */

add_action( 'admin_bar_menu', 'toolbar_link_to_mypage', 999 );

function toolbar_link_to_mypage( $wp_admin_bar ) {
	global $wp;
	$args        = array( 'action' => 'flush-rules' );
	$self        = $_SERVER["HTTP_HOST"] . $_SERVER["REQUEST_URI"];
	$current_url = add_query_arg( $args, $self );
	$args        = array(
		'id'    => 'dk_flush_network_permalink',
		'title' => 'Flush network permalink',
		'href'  => $current_url,
		'meta'  => array( 'class' => 'my-toolbar-page' )
	);
	$wp_admin_bar->add_node( $args );
}

add_action( 'init', 'dk_flushrules_init' );
function dk_flushrules_init() {

	if ( wp_is_large_network() ) {
		return;
	}
	if ( isset( $_GET['action'] ) && $_GET['action'] == 'flush-rules' ) {
// ...and we're probably still friends.
		$sites = wp_get_sites( array( 'network' => 1, 'limit' => 1000 ) );
		foreach ( $sites as $site ) {
			switch_to_blog( $site['blog_id'] );
			delete_option( 'rewrite_rules' );
			restore_current_blog();
		}
	}
}