A new Promotions screen under Admin manages discount campaigns: the code, which billing period it applies to, how long the discount lasts, when the offer ends, and how many times it can be redeemed.
Stripe still decides what is charged. A campaign records the Stripe promotion code, and checkout revalidates the code before the customer pays rather than trusting the link they arrived on.
A campaign link pre-applies its code, and the price shown always states what happens at renewal.