Is the 8.x-1.0-beta1 working?

Thanks for reporting this. I have tested the 8.x-1.0-beta1 with clean install of Drupal 10.3.1, Commerce 8.x-2.38 and Commerce Stock 8.x-1.3 and the \Drupal\commerce_stock_notifications\Plugin\Commerce\InlineForm\Notification::submitInlineForm() is executed as expected.

Do you have any other modules (contrib / custom) which can alter commerce or commerce stock somehow? I have tested this on PHP 8.1. Have not time to test it on Drupal 9.5. yet.

@introfini There is another similar module https://www.drupal.org/project/commerce_product_reminder →

Have you tested it?