I flew PRG-JFK(-LAX) on Delta yesterday (May 14) and was surprised when checking in that I was told I could visit the MasterCard lounge. I asked if I needed a pass but the check in agent just said to show my boarding pass.
So I did, fully expecting to be turned away (the Menzies Lounge across the way was a Priority Pass lounge so that was plan B) but was welcomed in. Perfectly fine lounge and way more comfortable than the holding pen post security...
I don't know if this is a change or they were just confused by the "PREM" on my boarding pass (as a DM).
As it turns out, I did receive an OpUp but while check in agent could have known that was coming, I doubt the lounge guardian would have.