I implemented the ‘Recurring charge discounts’ on a development store, using this guide (Recurring charge discounts (shopify.dev)) and managed to get a proper response.
Sorry for the delay, there is some ambiguity in “Subscriptions” as the team that handles billing subscriptions for app developers is different than for merchants who are selling subscriptions to customers. I had to track down the right team the help out.
Here is the response I got:
The most straightforward way to query for the discount applied to the subscription would be using the queries in the docs here and modifying the GQL query to also include the priceAfterDiscount.