Never had a guest cancel day-of, but if I did, I would send the guest a message "Hi XX, so when a guest reservation is cancelled day-of check-in, hosts and guests both get notifications to review (in case the guest did arrive and the place wasn't as advertised, so they left). Since you didn't stay, I have nothing to review or rate you on and just wanted to let you know I'm not planning to leave any review, and assume you won't either."
It is such an absurd thing. But no more absurd than when brand new hosts get warning messages about their stats being too low, when they haven't even hosted their first guest yet. Or being warned about being delisted due to a safety issue they won't disclose to you. Or being urged to unblock your calendar to get more bookings, as if you just blocked it for no reason.
It's just unbeliveable that they can even come up with stuff like this. I'd love to know who did.