The FOM does not specifically say "shirt with a collar" but all of the shirts listed as acceptable have collars.

Consevative, or casual dress shirts have collars. Oxford style button down shirts have collars. Dress camp shirts have collars. And yes, Turtlenecks and mock turtleneck shirts have a collar.

So yes, you need to wear a collared shirt here at Delta Air Lines, the same as it has been for decades.