Can I remind guests shortly before their table time?

Yes. Settings → Tables has a setting called Remind guests before their booking: off, 15 minutes before, or 30 minutes before. The guest gets a short WhatsApp with the table time, the party size and the booking number, and a line saying to reply if they are running late. It only reaches a guest who has messaged your restaurant on WhatsApp, the same rule as the confirmation. If you cancel the booking, mark a no show or seat them early, the reminder is cancelled with it. The Tables, tabs and the kitchen screen tab in the docs covers reminders.