Don't have an SCTA rule book handy, but I did google something about an "update to the 2016 SCTA rules for leathers". Presumably that's the 2017 change you mention -
7.C.2 LEATHERS
One-piece or two-piece 350 degrees (zipped together) all leather is required and the zipper must be sewn to the leather. The use of the leather manufacturer's engineered materials, such as stretchable Kevlar, perforations, or any other of the manufacturer's materials, in non-critical areas is permissible. In critical areas (knees, elbows, forearms, back, shoulder, hips, etc.), armor or two layers of leather is highly recommended. Riders of motorcycles in the fuel classes are recommended to wear Nomex underwear or something similar in nature. Suits of synthetic materials are not permissable. Fairings (speed humps) on the back of leathers will be allowed in partially streamlined classes only.
Here's the AMA rules -
2. P. (IV) LEATHERS
Suits made of synthetic material are not permissible. The use
of stretchable Kevlar and perforated materials in non-critical
areas are permissible. Leather suits shall be one-piece design
or joined by a full circumference zipper at the waist.
V16.1 25
Leathers cannot be too big or loose. Critical area (knees,
elbows, forearms, shoulders) armor or 2-layers of leather is
highly recommended. Due to invisible flames, riders? of
motorcycles burning fuels of Methanol content are
recommended to wear Nomex underclothing or something
of similar nature.
So it does appear that the regs are more or less the same these days and an SCTA approved suit does not have to be solid leather everywhere. I stand corrected.