This removes a parent menu link from its submenu in the WordPress admin.

remove-parent-link-from-submenu.php

<?php

// First, add the top-level menu page
// This adds the same link to the submenu when there are other submenu pages registered underneath it.
add_menu_page( esc_html__( 'Top Page', 'text-domain' ), esc_html__( 'Top Page', 'text-domain' ), 'manage_options', 'slug', 'callback', 'icon' );

// We don't need to have a link for the parent in the submenu, so this overwrites it
// However, this will leave an empty link item that's still visible due to padding
add_submenu_page( 'slug', '', '', 'manage_options', 'slug', 'callback' );

// This gets rid of the submenu item that overwrites the parent
// This effectively removes the parent link in the submenu
remove_submenu_page( 'slug', 'slug' );