If this isn't a glitch in the system, then one possibility that immediately comes to mind is:

*puts on Devil's Advocate's hat*

Delta simply wants to give priority for upgrades to non-connecting passengers because they generally represent a higher profit margin for Delta since their fares are usually higher to start with (the "hub" premium) while "using up" less resources (only a single seat on a single flight per leg) than connecting passengers.

If the report of Delta no longer displaying Standby Upgrade lists at the gate turns out to be more widespread/not also a glitch, then they may be doing all of this on purpose so that they can "manipulate" things whichever way they want without most people ever being any the wiser.

Now, all of the recent questions by FAs and CRC agents asking about how successful I've been with upgrades is starting to make some sense.....since it would seem that Delta has radically changed the way upgrades are cleared.

*removes hat*

Hopefully, these are nothing more than simply glitches due to the recent implementation of Journey Control that will eventually be "fixed".

If not......

A.J.