If it is.....it's news to me.
Earlier this year, I flew a NYC-SEA RT that was booked via Delta.com.
The only redeye return that didn't "break the bank" consisted of a SEA-IAD codeshare operated by AS (no longer
) with a connection to a DCA-LGA DL shuttle flight.
The connection didn't break the fare.
FWIW, switching airports on a Monday morning proved to be smoother than I thought it would be as I simply took the Washington Flyer/DC Metro combo to get from IAD to DCA.
In the past, I have also connected at BOS from mainline to the Shuttle.
Another indication I've occasionally noticed would be "connection info" displayed on the GIDS (usually info for only 1 or 2 flights) after disembarking a shuttle flight......at DCA, BOS and even LGA.
I've also recently started to notice some "suggested" routings (for paid and/or award tickets) that include a segment on the DL Shuttle when searching for flights on Delta.com.
A.J.