Am a DL Gold/STE+ too. Flew AF in Premium Economy last November and there was zero issue with me being recognized as a STE+ with AF. Had access to the Sky Priority check-in lines and AF lounges. My DL SM number is entered during the booking process.

Ironically I had trouble with a DELTA agent not recognizing me as a DL Gold/STE+ when a LAX SC lounge dragon initially wouldn't let me in because the AF boarding pass in Apple Wallet doesn't show STE+ status (the BP in the AF app does). She made me read out my SM number to confirm my DL status.