In my pre-AA days, I was platinum on Delta for many years, always qualifying with 100+ segments. Commuted a lot to ROC/BUF, and I usually routed it MIA-ATL-CVG-ROC/BUF.

More than once I got a call from DL asking me why I hadn't routed direct from ATL (I believe it was called the Revenue Protection Unit). While never directly threatened with loss of miles, they let me know they would "continue to monitor my account for possible mileage abuse." They may have even quoted a sentence from the SkyMiles program that said something to that effect.

Back then, DL gave F upgrades to Plat members at the time of booking, so it was easy to say "I liked the seats on that flight better".

Then, they implemented restrictions on auto upgrades on L, U, and T fares.
Second, they stopped their MIA-ATL meal service.
The final straw was when they added "S" fares to the restriction list

Combined with the other three fare codes, I knew what DL thought of us, we were just ..... (you figure it out).