I wanted to replace the text of my menu with font awesome icons so I created a new walker class to do replacements for links with specific outbound links. Not at all generic but I wanted to stash it. I'm sure there are better ways to do this.

_RoanokeMenuIconStyles.scss

a.social-link {
  * {
    &.color-orange {
      color: orange;
      &:hover {
        color: darken(orange, 10%);
      }
      &:visited {
        color: orange;
      }
    }
    &.color-mastodon {
      color: #2b90d9;
      &:hover {
        color: darken(#2b90d9, 10%);
      }
      &:visited {
        color: #2b90d9;
      }
    }
    &.color-twitter {
      color: #1da1f2;
      &:hover {
        color: darken(#1da1f2, 10%);
      }
      &:visited {
        color: #1da1f2;
      }
    }
    &.color-mail {
        color: #000;
        &:hover {
            color: darken(#000, 10%);
        }
        &:visited {
            color: #000;
        }
    }
    &.color-facebook {
      color: #3b5998;
      &:hover {
        color: darken(#3b5998, 10%);
      }
      &:visited {
        color: #3b5998;
      }
    }
    &.color-linkedin {
      color: #0077b5;
      &:hover {
        color: darken(#0077b5, 10%);
      }
      &:visited {
        color: #0077b5;
      }
    }
    &.color-instagram {
      color: #e1306c;
      &:hover {
        color: darken(#e1306c, 10%);
      }
      &:visited {
        color: #e1306c;
      }
    }
    &.color-flickr {
      color: #ff0084;
      &:hover {
        color: darken(#ff0084, 10%);
      }
      &:visited {
        color: #ff0084;
      }
    }
  }

}

RoanokeMenuExampleUsage.php

<?php

// https://developer.wordpress.org/reference/functions/wp_nav_menu/
$args = [
    'menu' => 'roanoke-menu-bar',
    'container' => 'nav',
    'container_class' => 'roanoke-menu-bar',
    'container_aria_label' => 'Menu Bar',
    'walker' => new RoanokeMenuWalker(),
];
wp_nav_menu($args);

RoanokeMenuWalker.php

<?php

// Requires 	wp_enqueue_script('font-awesome', '//use.fontawesome.com/releases/v5.8.0/js/all.js');
// to be part of your scripts
class RoanokeMenuWalker extends Walker_Nav_Menu {

	public static $replacements = [
		'https://instagram.com/artlung' => 'fab fa-instagram color-instagram',
		'https://xoxo.zone/@artlung' => 'fab fa-mastodon color-mastodon',
		'https://twitter.com/artlung' => 'fab fa-twitter color-twitter',
		'https://www.facebook.com/artlung' => 'fab fa-facebook color-facebook',
		'https://www.linkedin.com/in/artlung/' => 'fab fa-linkedin color-linkedin',
		'https://www.flickr.com/photos/artlung/' => 'fab fa-flickr',
		'mailto:joe@artlung.com' => 'fas fa-envelope color-mail',
		'https://artlung.com/feed/' => 'fas fa-rss color-orange',
	];

	public function start_el( &$output, $data_object, $depth = 0, $args = null, $current_object_id = 0 ) {
		parent::start_el( $output, $data_object, $depth, $args, $current_object_id ); // TODO: Change the autogenerated stub
		$href = $data_object->url;
		$inner_text = $data_object->title;
		foreach(self::$replacements as $needle => $replacementClass) {
			if (strpos($href, $needle) !== false) {
				$outputDom = new DOMDocument();
				$outputDom->loadHTML($output);
				$anchors = $outputDom->getElementsByTagName('a');
				foreach($anchors as $anchor) {
					if ($anchor->getAttribute('href') === $href) {
						$anchor->nodeValue = '';
						$fontAwesomeIcon = $outputDom->createElement('i');
						$fontAwesomeIcon->setAttribute('class', $replacementClass);
						$anchor->appendChild($fontAwesomeIcon);
						// set attribute on the anchor
						$anchor->setAttribute('class', 'social-link');
						$anchor->setAttribute('target', '_blank');
					}
				}
				$output = $outputDom->saveHTML();
			}
		}
	}

}