Create TWIG-filter token

/themes/contrib/bfd/templates/includes/footer.html.twig

wrap e-mail and encode

{# set email_footer variable #}
{% set email_footer = 'mailto:' ~  drupal_token('site:mail')  %}

{# value as variable #}
{% set encoded_mail =  email_footer | base64_encode  %}
{# Encodes mailto:e-mail to the set of symbols. #}

<i class="fas fa-envelope-open-text"></i> <span class="email">
<a href="javascript:window.location.href=atob('{{ encoded_mail }}')">{{ drupal_token('site:mail') }}</a>
</span>

span.email {
    -webkit-touch-callout: none;
    -webkit-user-select: none;
    -khtml-user-select: none;
    -moz-user-select: none;
    -ms-user-select: none;
    user-select: none;
}
.email a {
    color: #843c39;
    border: .5px dashed #fff;
    border-radius: 17px;
    box-shadow: 0 0 0 2px #050b0e, 2px 1px 4px 3px rgba(10, 10, 0, 0.5);
    text-shadow: -1px -1px #530707;
    padding-left: .2rem;
    padding-right: .2rem;
}

base64_encode - is our new Twig-variable (must be declared)

How to inject base64_encode php-function?
(PHP 4, PHP 5, PHP 7, PHP 8)
base64_encode — Encodes data with MIME base64
base64_encode(string $string): string
Encodes the given string with base64.
This encoding is designed to make binary data survive transport through transport layers that are not 8-bit clean, such as mail bodies.
Base64-encoded data takes about 33% more space than the original data.

Extend Twig Extensions

1. Create directory base64_coding in /modules/contrib

mkdir /modules/contrib/base64_coding

tree
.
├── base64_coding.info.yml
├── base64_coding.services.yml
└── src
   └── TwigExtension
       └── Base64.php

2. base64_coding.info.yml

name: Custom Twig Filter - Base64 Coding
type: module
description: 'Provides text coding | decoding'
package: Core
core: '8.x'
core_version_requirement: ^8 || ^9 || ^10

3. base64_coding.services.yml

services:
 base64_coding.twig_extension:
   class: Drupal\base64_coding\TwigExtension\Base64
   tags:
     - { name: twig.extension }
   arguments:
     - '@entity_type.manager'

4. /src/TwigExtension/Base64.php

<?php

namespace Drupal\base64_coding\TwigExtension;

use Twig\Extension\AbstractExtension;

use Twig\TwigFilter;

/**
* MyTwigExtension Base64 provides function and filter.
*/
class Base64 extends AbstractExtension {
   public function getFilters()
   {
       return [
           new TwigFilter('base64_encode', 'base64_encode'),
           new TwigFilter('base64_decode', 'base64_decode'),
       ];
   }
}

Enable newly created module