When you book an appointment for a contact, they can get two emails: a “Confirmed: your appointment…” email right away (or an “Updated” one if you change the time), and a reminder 24 hours before. Before this fix, the checkbox on the appointment form only stopped the reminder, so contacts still got the confirmation email after you unchecked it.
The checkbox now reads Send the contact a confirmation email and a reminder 24 hours before. Uncheck it to stop both emails for that appointment, including the “Updated” email when you change the time. It still starts checked, so appointments you don’t change keep sending both emails as before.
The 24-hour reminder emails setting under Settings → Appointment Emails still controls only the reminder. Your own appointment notifications and SMS reminders are not affected.