I was cutting it close. I had a flog by today from LGA to DCA at 2:15, a pretty last-minute booking. I expected to get to the gate about 20 minutes before departure, but I only made it at 2:09, and missed the flight.

Why did I think I might still get on? Because Delta still advertises a 5-minute boarding cutoff for the Delta Shuttle: https://www.delta.com/us/en/check-in-security/check-in-time-requirements/delta-shuttle-check-in

And it looks like Delta still considers DCA-LGA (and vice versa) a Delta Shuttle route: https://www.delta.com/content/www/en_US/traveling-with-us/where-we-fly/flight-partners/delta-shuttle.html

I was unequivocally at the gate more than five minutes before departure, but the flight left early, at 2:10.

If the Shuttle is gone and the boarding cut-off was T-15, okay. I think Delta should make that clear. (My boarding pass showed boarding start and departure, but not closing.)