Hi there, @Anonymous.
Thanks for visiting the Community. I understand the need to have this feature available in QuickBooks. At this time, we don't have the turnaround as to when will this be added. Rest assured that your feedback is noted and will be submitted to our engineers for further study.
In the meantime, you can utilize this article: Apply a discount on an invoice or sales receipt. It helps you learn about how to turn on and use the discount feature so you can add it to an invoice or sales receipt.
You can also visit our QuickBooks Online Blog site to get updates on our latest news and product enhancements.
Always know the Community is around if there's anything else that you need.