This is a tiny Magento module that makes a 301 redirect to the product page when a SKU is passed to it's controller: https://example.com/gotosku/?sku=example-sku. If the product can't be found a 404 page will be shown instead.
The easiest way to install the extension is to use Composer, just run the following commands:
$ composer require kodbruket/magento2-gotosku
$ bin/magento module:enable Kodbruket_GoToSku
$ bin/magento setup:upgrade