Thanks for all your comments.

The flight was from the west to east coast.

I looked over the logs at flightstats. I found an entry that says, "Estimated Gate Departure" was changed to 11:55AM.

The notification I received was not about the change in departure time though. It said exactly this:

"Fly Delta

Today at 11:55AM
<small picture of boarding pass in Apple wallet on the right>
"

I did not receive any notification about boarding.

I received the above same notification when I took the flight next day, but with the original time 12:05PM. This time I did get notification about boarding.

I wonder if the above notification is not directly from the Delta app itself, but from the Apple wallet to which I had added my boarding pass from the Delta app.

*sigh* Please check your Android phone next time you fly.
FWIW, I have always had Android phones. I'm currently on an S9+ and had an S7 Edge up until last month. Both phones displayed the boarding time, including today on my PHX-SEA flight. kthxbye

Now on I'm always going to check the monitors.

My apple watch gets a notification as soon as those needing extra help board. I found that works at CDG as well as MSP or other US airports.

Cruising for a Bruising
 

YES. And it even depends on what version of Android your phone or tablet is using. My tablet routinely shows old (over a year old) notifications form the app.

Anyone who depends on Delta IT to get to the gate on time is cruising for a bruising.