Answer:
The ? icon above the expiration date is a bit tricky to understand. What it means is that an appointment that is booked after the expiration date will not be allowed to have the coupon code to apply. This is referring to physically booking the appointment on a date past the expiration of the coupon code's expiration date.
If you are booking the appointment while being within the time frame of the coupon codes lifespan, clients can book appointments in the future, because by default that coupon code is still active, regardless of the appointment being in the future.
So, for example, I set a coupon code to expire on June 23rd, and I book an appointment today (June 21st), for an appointment on June 24th, because I actually booked the appointment before it's expiration date, it will allow the coupon code to apply.
As long as the appointment is booked during the lifespan the coupon code is active, it doesn't matter if the appointment occurs past the expiration of the coupon.
Because the clients physically book now.
So, to test this you can set the coupon to have expired yesterday, try to book an appointment and apply the code and you'll get an error.
What the documentation refers to is the date of Scheduling the appointment, not the scheduled date of the appointment.