WooCommerce Subscriptions check if the logged-in user has any or given subscription tiers

is_subscriber.php

<?php
/**
 * Check for one or many WooCommerce Subscriptions by SKU
 *
 * @param string|array $sku_list Optional. Single SKU as string or array of SKUs to check against.
 * @return bool True if the user has an active subscription (matching the SKU(s) if provided), false otherwise
 */
function is_subscriber( $sku_list = null ){
    $user_id = get_current_user_id();

    if( ! $user_id ){
        return false; // Not logged in
    }

    $subscriptions = wcs_get_users_active_subscriptions( $user_id );

    if( empty( $subscriptions ) ){
        return false; // No subscriptions
    }

    if( $sku_list === NULL ){
        return true; // Early return if no SKUs to check
    }

    if( ! is_array( $sku_list ) ){
        $sku_list = [ $sku_list ]; 
    }

    foreach( $subscriptions as $subscription ){
        foreach( $subscription->get_items() as $item ){
            $product = $item->get_product();

            if( $product && in_array( $product->get_sku(), $sku_list ) ){
                return true; // If the params' array include an SKU, we grant access
            }
        }
    }
  
    return false;
}

function register_is_subscriber() {
    // As best as I understand, this allows global use of this function
}
add_action( 'init', 'register_is_subscriber' );