Adds back customizer link to Appearance in admin and admin bar when using the Full Site Editing in Gutenberg due out in WP 5.8

add_customizer_back_to_fse.php

<?php
/**
 * Plugin Name: WCD Add Customizer Back
 */

function wcd_add_customizer_back()
{
    if (!gutenberg_is_fse_theme()) {
        return;
    }
    if (is_admin() || current_user_can('edit_theme_options')):
        global $submenu;
        $customize_url = add_query_arg(
            'return',
            urlencode(wp_unslash($_SERVER['REQUEST_URI'])),
            'customize.php'
        );
        add_submenu_page(
            'themes.php',
            __('Customize'),
            __('Customize'),
            'customize',
            esc_url($customize_url),
            ''
        );
    endif;
}

function add_toolbar_items($admin_bar)
{
    if (!gutenberg_is_fse_theme()) {
        return;
    }
    if (is_admin() || current_user_can('edit_theme_options')):
        $customize_url = get_admin_url('', 'customize.php');
        $admin_bar->add_menu(array(
            'id' => 'customize',
            'title' => 'Customize',
            'href' => $customize_url,
            'meta' => array(
                'title' => __('Customize'),
            ),
        ));
    endif;
}
if (function_exists('gutenberg_is_fse_theme')) {
    add_action('admin_bar_menu', 'add_toolbar_items', 100);
    add_action('admin_menu', 'wcd_add_customizer_back', 999);
}

Updated to add in the admin bar and also check that user has edit_theme_options rights

Just tidied up a bit so as to check that the gutenberg full site editing is actually installed