Workaround for WooCommerce get_european_union_countries( 'eu_vat' ) after WooCommerce 4.0.0

woocommerce_get_european_union_countries_after_wc4.php

<?php
//See here why this is needed: https://github.com/woocommerce/woocommerce/issues/26105
function woocommerce_get_european_union_countries_after_wc4( $type = '' ) {
	$countries = WC()->countries->get_european_union_countries();
	if ( 'eu_vat' === $type ) {
		$countries[] = 'MC';
		$countries[] = 'IM';
		$countries[] = 'GB'; //Great Britain is still part of the EU VAT zone
	}
	return $countries;
}

//Usage to get the EU countries
$countries = woocommerce_get_european_union_countries_after_wc4();

//Usage to get the countries that are part of the EU VAT zone
$eu_vat_countries = woocommerce_get_european_union_countries_after_wc4( 'eu_vat' );