You're right - many of those have been re-branded as First (though they still keep the Business Class fare codes (J, Z, I).
That wasn't always the case, but it's a good change. Will keep the confusion down.
Interestingly, when searching on Matrix (and perhaps others) those flights are still marketed as Business. That's where I do most of my initial searches so have never really noticed the change at Delta.com.
An example is SLC-MEX which sells a Z fare:
Salt Lake City (SLC) to Mexico City (MEX) — Sat, Apr 8
Delta 641
Dep: 9:40 am
Arr: 2:35 pm
3h 55m
Airbus A319
Business (Z)