Hide protected post links from unregistered users

If anyone else needs it, I resolved it this way:

1. Created a new WordPress child theme
2. Added functions.php with the following content:

<?php

if ( class_exists('SimpleWpMembership') ) {
    function swpm_get_protected_posts() {
        global $wpdb;
        $protected_posts = array();

        if (!is_admin()) {
            $level_id = -1;
    
            $auth = BAuth::get_instance();
            if ($auth->is_logged_in()) {
                $level_id = $auth->get('membership_level');
            }
    
            $results = $wpdb->get_results("SELECT id, post_list FROM {$wpdb->prefix}swpm_membership_tbl");
    
            $allowed_posts = array();
            $all_posts = array();
            foreach ( $results as $result ) {
                if ( isset($result->id) && isset($result->post_list) ) {
                    $posts = (array) unserialize($result->post_list);
                    if ($result->id == $level_id)
                        $allowed_posts = $posts;
                    else
                        $all_posts = array_merge($all_posts, $posts);
                }
            }
            $all_posts = array_unique($all_posts);
    
            $protected_posts = array_diff($all_posts, $allowed_posts);
        }

        return $protected_posts;
    }

    add_action('pre_get_posts', 'swpm_exclude_protected_posts');
    function swpm_exclude_protected_posts($query) {
        $protected_posts = swpm_get_protected_posts();
        if (!empty($protected_posts)) {
            $query->set('post__not_in', $protected_posts);
        }
    }
    
    add_filter('get_previous_post_where', 'swpm_filter_adjacent_posts');
    add_filter('get_next_post_where', 'swpm_filter_adjacent_posts');
    function swpm_filter_adjacent_posts( $where ) {
        $protected_posts = swpm_get_protected_posts();
        if (!empty($protected_posts)) {
            $where .= " AND p.ID NOT IN (" . implode(',', $protected_posts) . ")";
        }
        return $where;
    }
    
}

This code adds two hooks:
1. pre_get_posts hook allows to exclude restricted posts from menus, category listings, etc.
2. get_previous_post_where and get_next_post_where excludes restricted posts from ‘Previous Post’/’Next Post’ links