Not too sure about that but I can say that I use deliveroo at least once a week and I'm a premium member (my country's having a 1€ / month deal because we're in lockdown). I receive e-mails from them regularly where a link will directly add a voucher (usually 10€ if you order for at least 20) on my account.

Tip : the public vouchers that deliveroo offer to their users for promotional events are very nice but it depends on your country and deliveroo partners. I would recommend to follow them on Twitter and activate the bell notification. It's very easy to miss a voucher code since it's limited to the 10-30k people and you might not check your email account in time. You don't have to order right away, just add the voucher code in your credit account so that if you want to order at the last minute, you still can use the discount.

The other day, I had a public voucher code stack on top of a random one I received and it gave me 20€ off my order.