You can still get into a Delta lounge as an Amex cardholder, but Delta has been tightening up its lounge access generally for a while. The biggest change was introducing two-tier membership; Amex cardholders, and even Delta's own top-level frequent flyers, now only get the base lounge membership (or equivalent). This shows up in things like being charged $29/person to bring guests with you, where previously you got two guests for free.