I can confirm that the JFK Clubhouse still lets Amex Plat cardholders enter. My card was put into a card reader (I guess to check that it's still valid) and both our boarding passes were scanned. I came with one guest on our way back to IND, entered around 6 pm and it was pretty full, maybe 2-3 tables were still empty.
The orders from the online menu were served promptly - drinks within 5 minutes, foods within 10. The cosmo was too sour, but someone else here mentioned that the cocktails were a little off.
On the (long) way to B43 I noticed the queue outside the SkyClub. I didn't check the Centurion lounge as I've been there in Dec - the Clubhouse is the best choice of all three.