Stock status label module allow you to show a custom availability info label in product page.
Run command below from your magento2 root.
composer require lucasart/stock-status-label:~1.2.0
php bin/magento setup:upgrade
Enable and configure the module in admin.
Stores -> Configuration -> Catalog -> Inventory -> Stock Status Label
That's all! Clean magento2 cache and go to product page in frontend. You will see the stock status label.