SMS would NOT let me SDC a LGA-BOS Shuttle flight as a GM a couple months ago. It could be that the agent that I talked to was wrong, but I didn't have time to argue or escalate (as I had to pack in a hurry to just make it to my flight; I'd been planning on taking a later one.)

This was a mid-day mid-week flight. There was plenty of space on the flight I wanted to SDC onto, and pretty much all flights other than possibly the peak commuting hour ones (which were well past when I'd have wanted to leave.)

Sent an email to Delta to complain since the website says you can SDC a Shuttle flight (specifically on the GM/PM SDC waive/Standby info page), but didn't get a reply.