Canon abandoned their DO element (PF in Nikon term) from the high-end lenses, possibly because of the flare issue which is inherent and cannot be solved by the lens coating.
I guess that Sigma decided to stay away from the diffraction optics from the outset. Apparently, both Canon and Nikon are abandoning optical designs using PF elements.
The new Sigma 200/2.0 doesn't accept TC, which may disappoint some potential buyers a bit.