EF said the inventory is still being published to the GDS... DL just doesn't want EF to show it.

The network traffic argument is a load of crap, even assuming the GDS is configured to send those requests to DL's system. If that's the real reason, DL wouldn't want EF to show availability displays for revenue fare classes either.

As others have mentioned, this is clearly about reducing the information available to customers make the upgrade process far more opaque, and in doing so, drive revenue (or minimize "lost" revenue, if you prefer).

Perhaps it's time to investigate what's involved in obtaining a GDS subscription.