There's an old FT post that mentions seeing text on the DL website, but that post is almost 3 years old. I think it's a been at least 2 years since the text has been removed.
It's not hard to find in the reserve t&c's:
Upgrade Priority on Delta
Increased Upgrade Priority within Medallion tier and fare class is applicable only to primary Card Member and within the Complimentary Medallion Upgrade Program. Airport Paid Standby Program and Mileage Award Upgrades are not eligible.
https://www304.americanexpress.com/p...4#cardbenefits