Do customers need to provide an email address when they subscribe to a product?

If customers opt to receive a notification for an out-of-stock product, do they need to give out their email addresses when they fill out the subscribe form?

No. The Subscribe Form is just a simple click of a button and does not require complicated steps or ask you for any personal information just to be able to receive a notification about the desired product's availability. The app will only send browser notifications to customers who agreed to subscribe to your store's back in stock notifications. The app currently does not have the capability to send email notifications.