WordPress functions to fetch external JSON (and store in transient + option backup) and also functions to setup programmatic URL patterns and use custom template files for said URLs

functions.php

function get_cached_external_json($url) {
    $transient_key = 'our_fetched_json_' . md5($url);
    $option_key = 'our_fetched_json_backup_' . md5($url);
    
    $cached_data = get_transient($transient_key);
    
    if (false !== $cached_data) {
        return $cached_data;
    }

    $response = wp_remote_get($url, array(
        'timeout' => 15,
        'headers' => array(
            'Accept' => 'application/json'
        )
    ));

    if (is_wp_error($response)) {
        // Return last known good data from options if available
        $backup_data = get_option($option_key, false);
        if ($backup_data) {
            return $backup_data;
        }
        return false;
    }

    $body = wp_remote_retrieve_body($response);
    $data = json_decode($body, true);

    if (null === $data) {
        // Return last known good data from options if available
        $backup_data = get_option($option_key, false);
        if ($backup_data) {
            return $backup_data;
        }
        return false;
    }

    // Set transient for x amount of time, measured in seconds
    set_transient($transient_key, $data, 8 * HOUR_IN_SECONDS);

    // Store a persistent backup in options (with autoload disabled)
    if (false === get_option($option_key, false)) {
        add_option($option_key, $data, '', false);
    } else {
        update_option($option_key, $data, false);
    }

    return $data;
}

///////////////

function custom_repo_rewrite_rule() {
    add_rewrite_rule('^repo/([^/]+)/?', 'index.php?custom_repo=$matches[1]', 'top');
}
add_action('init', 'custom_repo_rewrite_rule');

function custom_repo_query_vars($query_vars) {
    $query_vars[] = 'custom_repo';
    return $query_vars;
}
add_filter('query_vars', 'custom_repo_query_vars');

///////////////

function custom_repo_template_include($template) {
    if (get_query_var('custom_repo')) {
        return get_template_directory() . '/repo-template.php';
    }
    return $template;
}
add_filter('template_include', 'custom_repo_template_include');