Android - Disable "Upcoming Alarm" Notification (without disabling alarm)
After spending an hour on the phone with Google tech support (don't know why I didn't think of this earlier) their representative was able to replicate the issue.
At this time there appears to be no way to disable this notification on a stock/unrooted Pixel. I'll update the answer if I get any further information.
A long press 'upcoming alarms' will bring up another screen where you can customize the 'upcoming alarms' notification category.
Well, this is a late response but I just ran across this as I am having the same problem. Thank to jszpila here for posting Google's response so I didn't have to waste any more time trying to get rid of the "upcoming alarm" notification.
As a work-around, I changed the sound for the "upcoming alarm" notification to "None". The notification still comes up but it is unobtrusive.