They don't have a formal policy, but there was this one time I forgot to set an alarm and woke up just after my flight out of Newark 30 miles away had left. Called AS, they put me on standby at the airport, got my BP when I checked in for the evening flight to SEA. I have never been happier to be in a middle seat on a transcon ever (I was going to have to burn a LOT of miles for a last seat Y ticket instead). This may have been before the formal "we cancel on no-shows" rule (this was like 2014 or so), but I had for sure missed my flight and no-showed it.